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"
                }
            }
        ]
    }
}