Karabiner-Elements をインスコして `~/.config/karabiner/assets/complex_modifications` に突っ込む

fnCaps2HanZen.json

{
  "title": "Fn＋CapsLockキーを英かなトグルにする",
  "rules": [
    {
      "description": "US入力と、ことえりのひらがなをトグルする",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "caps_lock",
            "modifiers": {
              "mandatory": [
                "fn"
              ]
            }
          },
          "to": [
            {
              "select_input_source": {
                "input_source_id": "^com\\.apple\\.inputmethod\\.Kotoeri\\.Japanese$"
              }
            }
          ],
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "en"
                }
              ]
            }
          ]
        },
        {
          "type": "basic",
          "from": {
            "key_code": "caps_lock",
            "modifiers": {
              "mandatory": [
                "fn"
              ]
            }
          },
          "to": [
            {
              "select_input_source": {
                "input_source_id": "^com\\.apple\\.keylayout\\.US$"
              }
            }
          ],
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "ja"
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}