Good fun! Not sure if you’re still maintaining this or not, but somewhere along the line GMS2 changed the way depth works, causing the skybox to get drawn over half of the screen. (It looks like someone else had a similar problem, but I don’t know if it was caused by the same issue or not).
Anyway, when you draw the skybox, you now need to force the depth buffer off (gpu_set_zwriteenable, as well as gpu_set_ztestenable if you really want) to prevent the background from persisting in 3D space after it’s been drawn. Not sure when GameMaker changed this, since it works just fine in GMS1. ¯_(ツ)_/¯
Also, I’m impressed that you managed to model everything in the game using exclusively paths. I would have just gone straight to 3D object files, lol.