Alright! I managed to completely swap out the inefficient Scene Tile instantiation system with a GridMap implementation.
I had to completely remodel the floor and wall tiles. I also deleted some faces that weren't being used.
Here is the new specs! The upper Godot instance is actually the editor. I can't think of much else I can do at the moment so I'll just upload the build as soon as I can.
Although the memory is still at 200~ MB...