Uau, that is a case!!
My only complain is that the player´s position is too off screen when going down a hill, but I guess that is an easy adjustment to what you have so far and that such adjustment needs the actual player.
I´m not sure you need a 3d package yet, your player could be a 3d box that you can easily render out of glb itself saved as bmp.
If those sprites match, then you have a good thing going.
Again, I´d advise that the floor ground should always be visible.
You did see the isle gamebuino game I talked about before right?