Thanks so much for the response! I'll study this when I get back into it, though there was another thing that was worrying me - I have two GMS2 windows open, one that's just this imported project, and my game which I've imported all the scripts/objects to. I rearranged my tilesets to look like the ones in the demo (6-wide floor, then 6-wide wall below it) just to see if it'd work.. but it's black. My character renders, doors appear, but the background is black.
Not sure if there's something in my code that's causing this, but I can't figure out for the life of me why none of the tiles are rendering with no changes whatsoever lol. The same code works fine in the actual demo project.
If I can figure this out.. and get it to work with the legacy, it'd be a lot easier to start moving it over to the new layered system because I'd be able to see it working..