Make a copy of your map and straight up delete a bunch of stuff from it. Imports, destructables, terrain, resetting OE data, resetting gameplay constants, etc.. See if it loads. If no, repeat and delete another bunch of stuff. When you finally delete whatever stops it from loading in Reforged, backtrack and think about what you deleted. Try to narrow it down.
If you save new copies along the way it'll be easier to figure out what it is. I would recommend a first pass just deleting/resetting almost everything just to see if you can confirmed get it to load.