I actually used draw_sprite_pos() and the view rotation is the fake z axis. When you scroll up and down it actually changes the view height while keeping the port the same size. So I also had to turn on and off the box sprites depending on the view rotation. The depth sorting works pretty much like depth=-y but it uses the ypos relative to the screen rotation.
You have more patience than me. I tried doing something similar to that like year ago and getting frustrated with that was what prompted me to start getting more familiar with vertex buffers and 3d space stuff.
Since you're using draw_sprite_pos() functions, does this only look right with flat shaded sprites? Like would a brick texture have shearing issues?
From what I've tried so far I haven't had any trouble with texture shearing, there is an asset I saw on the marketplace that seems it would fix it though if it became an issue https://marketplace.yoyogames.com/assets/4720/draw-sprite-pos-fixed