Can an Optifine CIT texture have a different texture in the GUI vs in the player's hand?

I was wondering if there was a way to make an item have a different texture in the GUI than the player’s hand with normal optifine.

For example, I have a long staff item I am trying to add, which is a 32×32 texture. I’ve scaled it to 2x in the 3rd person and on the ground, so it looks very nice, and the pixels are the same size as 16×16 items. I can scale it up in the GUI as well, but with that it protrudes outside of the GUI box and doesn’t look very good. I have an icon I would like to use in the GUI but I don’t know how to make it apply:

Here’s how it looks in 3rd person, as well as a comparison of how it should look vs how it actually looks.

Here is my model.json to show what I’ve tried:

{
    "parent": "builtin/generated",
    "textures": {
        "layer0": "./staff",
        "layer1": "./staff_icon"
    },
    "display": {
        "thirdperson": {
            "texture": "layer0",
            "rotation": [ 0, 90, -35 ],
            "translation": [ 0, 0.25, 2 ],
            "scale": [ 2, 2, 1 ]
        },
        "firstperson": {
            "texture": "layer0",
            "rotation": [ 0, -135, 25 ],
            "translation": [ 3, -5, -1 ],
            "scale": [ 3.4, 3.4, 1.7 ]
        },
        "gui": {
            "texture": "layer1",
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 0, 0 ],
            "scale": [ 2.0, 2.0, 1.0 ]
        },
        "ground": {
            "texture": "layer0",
            "rotation": [ 0, 0, 0 ],
            "translation": [ 0, 0.5, 0 ],
            "scale": [ 2.0, 2.0, 1 ]
        }
    }
}

However with this, they both get overlayed on top of each other no matter where they are, and it looks even worse than before:

It’s because lines like "texture:", "layer0" don’t actually do anything. I want it so that in the GUI it uses staff_icon.png to display the textures, but in all other places it displays the normal staff.png and looks good.

Is there any way to make a certain situation, GUI in this case, hide or display certain layers of the model? If not, then I’m clearly misusing layers, so what are they really for?