I'm glad I can help! Regarding the weld cooldown, yeah there was a reason I put "perhaps" there.
I think the main problem with the 5-door level (aside from how incredibly tedious it is) is that it just feels like 5 separate normal puzzles, probably catalyzed to some extent by the fact that having multiple yellow doors in one level has only one level associated with it, if I remember correctly.
If you intend for the later levels to have no boxes, I would think it'd make more sense to have useless key parts scattered around than wooden boxes; maybe roundish chunks of metal, idk.
Unrelated, but maybe the red doors should have something to visually distinguish them from the yellow doors other than color? I'm not exactly sure how hard colorblindness would make this game right now, but it couldn't hurt.