Even if I delete them entirely, it’s still a disco-ceiling.
At this point I think I’ve tried everything and I have to emphasize again, nothing I do even changes it, so it’s not as though I’m able to tweak it (with the exception of setting roughness to 1.0).
I’m gonna have to conclude that Unreal can’t handle roughness on large surfaces or there’s a bug. I’m using 5.3.2