the resourcepack doesn't work. I try to use it but it doesn't work

How to customize the resourcepack?

Your datapack idea is great but not implemented efficiently.

With your version of it, you are using the /item command that only appeared in 1.17 so you can't claim your datapack is 1.14 compatible.
A better way (in my opinion) to achive your goal is to run a function as all player that triggered the score.
In this function, you can copy player score to a storage and then use an item_modifier to modify the item CustomModelData from the storage by using the copy_nbt tag.

It would look something like this :
tick.mcfunction : execute as @a[scores={rp_ApplyModel=1}] run function rp_items:apply_model
apply_model.mcfunction : execute store result storage temp CMD int run scoreboard players get @s rp_ApplyModel
item modify entity @s weapon.mainhand rp_items:apply_model
apply_model.json :
[
  {
    "function": "minecraft:copy_nbt",
    "source": "minecraft:temp",
    "ops": [
      {
        "source": "CMD",
        "target": "CustomModelData",
        "op": "replace"
      }
    ]
  }
]
I may have made syntax mistakes so feel free to ask if something isn't working properly.
With these changes, you will be able to use any number as a CustomModelData without having to hardcode each and every single value.
I hope this will help you.