This is my Karabiner config, which provides a rough home-row-mod-like experience. The rules that follow the "<key>/<mod>" naming convention might help others write similar or better rules. "to_if_alone" doesn't perfectly replicate mod-tap behavior though. If you hold down a key and release it, the key will still register as it's "to_if_alone" va…

karabiner.json

{
    "global": {
        "ask_for_confirmation_before_quitting": true,
        "check_for_updates_on_startup": true,
        "show_in_menu_bar": true,
        "show_profile_name_in_menu_bar": false,
        "unsafe_ui": false
    },
    "profiles": [
        {
            "complex_modifications": {
                "parameters": {
                    "basic.simultaneous_threshold_milliseconds": 50,
                    "basic.to_delayed_action_delay_milliseconds": 500,
                    "basic.to_if_alone_timeout_milliseconds": 1000,
                    "basic.to_if_held_down_threshold_milliseconds": 500,
                    "mouse_motion_to_scroll.speed": 100
                },
                "rules": [
                    {
                        "description": "A/CTRL",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "a",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "left_control"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "a"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "S/ALT",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "s",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "left_alt"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "s"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "D/GUI",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "d",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "left_command"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "d"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "F/SHIFT",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "f",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "left_shift"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "f"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "SEMI/CTRL",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "semicolon",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "right_control"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "semicolon"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "L/ALT",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "l",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "right_alt"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "l"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "K/GUI",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "k",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "right_command"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "k"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    },
                    {
                        "description": "J/SHIFT",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "j",
                                    "modifiers": {
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "right_shift"
                                    }
                                ],
                                "to_if_alone": [
                                    {
                                        "key_code": "j"
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    }
                ]
            },
            "devices": [
                {
                    "disable_built_in_keyboard_if_exists": false,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": true,
                        "is_pointing_device": false,
                        "product_id": 834,
                        "vendor_id": 1452
                    },
                    "ignore": false,
                    "manipulate_caps_lock_led": true,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": false,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": false,
                        "is_pointing_device": true,
                        "product_id": 834,
                        "vendor_id": 1452
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": true,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": true,
                        "is_pointing_device": true,
                        "product_id": 8963,
                        "vendor_id": 4617
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": true,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": true,
                        "is_pointing_device": false,
                        "product_id": 8963,
                        "vendor_id": 4617
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": true,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": false,
                        "is_pointing_device": true,
                        "product_id": 8963,
                        "vendor_id": 4617
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": false,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": false,
                        "is_pointing_device": true,
                        "product_id": 613,
                        "vendor_id": 1452
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                },
                {
                    "disable_built_in_keyboard_if_exists": false,
                    "fn_function_keys": [],
                    "identifiers": {
                        "is_keyboard": false,
                        "is_pointing_device": true,
                        "product_id": 613,
                        "vendor_id": 76
                    },
                    "ignore": true,
                    "manipulate_caps_lock_led": false,
                    "simple_modifications": [],
                    "treat_as_built_in_keyboard": false
                }
            ],
            "fn_function_keys": [
                {
                    "from": {
                        "key_code": "f1"
                    },
                    "to": [
                        {
                            "consumer_key_code": "display_brightness_decrement"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f2"
                    },
                    "to": [
                        {
                            "consumer_key_code": "display_brightness_increment"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f3"
                    },
                    "to": [
                        {
                            "apple_vendor_keyboard_key_code": "mission_control"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f4"
                    },
                    "to": [
                        {
                            "apple_vendor_keyboard_key_code": "spotlight"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f5"
                    },
                    "to": [
                        {
                            "consumer_key_code": "dictation"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f6"
                    },
                    "to": [
                        {
                            "key_code": "f6"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f7"
                    },
                    "to": [
                        {
                            "consumer_key_code": "rewind"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f8"
                    },
                    "to": [
                        {
                            "consumer_key_code": "play_or_pause"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f9"
                    },
                    "to": [
                        {
                            "consumer_key_code": "fast_forward"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f10"
                    },
                    "to": [
                        {
                            "consumer_key_code": "mute"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f11"
                    },
                    "to": [
                        {
                            "consumer_key_code": "volume_decrement"
                        }
                    ]
                },
                {
                    "from": {
                        "key_code": "f12"
                    },
                    "to": [
                        {
                            "consumer_key_code": "volume_increment"
                        }
                    ]
                }
            ],
            "name": "Default profile",
            "parameters": {
                "delay_milliseconds_before_open_device": 1000
            },
            "selected": true,
            "simple_modifications": [],
            "virtual_hid_keyboard": {
                "country_code": 0,
                "indicate_sticky_modifier_keys_state": false,
                "mouse_key_xy_scale": 100
            }
        }
    ]
}