minecraft_lzstools/assets/minecraft/items/iron_pickaxe.json

{
  "model": {
    "type": "select",
    "property": "custom_model_data",
    "fallback": {
      "type": "minecraft:model",
      "model": "minecraft:item/iron_pickaxe"
    },
    "cases": [
      { "when": "lzstools:hammer", "model": { "type": "model", "model": "lzstools:items/hammer/hammer" } }
    ]
  }
}