Sample config for the Minecraft mod jsonRecipeEdit, implements starting inventory in virtual command chains, see giveinitialinv chain to update for your modpack

Starting inventory

[
  "** Start with some basic gloal settings **",
  {
    "command": "virtual command chain",
    "trigger": "ServerStart",
    "name": "ServerLoad",
    "chain": [
      "jsonRecipeEdit set server_init"
    ]
  },
  "** First load: set gamerules **",
  {
    "command": "virtual command chain",
    "trigger": "OnSet: server_init",
    "log": true,
    "name": "InitWorld",
    "chain": [
      "gamerule logAdminCommands false",
      "gamerule commandBlockOutput false"
    ]
  },
  "** First load: initial inventory scoreboard **",
  {
    "command": "virtual command chain",
    "trigger": "OnSet: server_init",
    "name": "InitInitialInv",
    "chain": [
      "scoreboard objectives add initialinv dummy"
    ]
  },
  "** Trigger when a player joins**",
  {
    "command": "virtual command chain",
    "trigger": "PlayerJoin",
    "dim": "*",
    "filter": "",
    "name": "initialinv_init",
    "chain": [
      "scoreboard players add @a[r=-1] initialinv 0",
      "testfor @p[score_initialinv=1,r=-1]",
      "jsonRecipeEdit run giveinitialinv",
      "scoreboard players set @p[score_initialinv=1,r=-1] initialinv 2"
    ]
  },
  "** Here is where you actually give the player the **",
  "** initial inventory **",
  {
    "command": "virtual command chain",
    "trigger": "Run: giveinitialinv",
    "dim": "*",
    "name": "InitialInv",
    "chain": [
      "give @p[score_initialinv=1,r=-1] minecraft:stick 3",
      "give @p[score_initialinv=1,r=-1] minecraft:sapling 1",
      "give @p[score_initialinv=1,r=-1] minecraft:dirt 1",
      "give @p[score_initialinv=1,r=-1] minecraft:dye 32 15"
    ]
  }
]