Well, like MANY AVNs on Itch, this game uses HxS2 assets, this is an asset set ripped from the HoneyXSelect game engine. The Dev has little power over that. Skin tone, eye colour and size, avatar size, bust size, overall dress size, butt size.... these things can be managed to some degree... BUT, the basic models skeleton, the meshed frame, verticies, the skinned nodes, etc etc etc that is MUCH harder to change and requires A LOT more work.
You move from animation, and coding, in to the realm of high end 3d modelling and CAD, and high end graphic design and texturing. Doing those properly, can in many cases can actually be MUCH harder than coding or animating.
In fact, if a developer is going to modify assets to that degree and employ those skills, if the engine supports it, it is often better to make your own, proprietary models.
No one is going to do that for a side project.