It appears that I only had to remove some minor statistics-related mods and the GuP wallpaper loading screens for 8.3 to work. Everything else seems to still function.
Edited a few minutes later: Locastan's statistics messages has been patched for 8.3, so that's out of the way. Just the wallpapers then.
Edit on top of the other edit: Got the login wallpapers to work. Must have misplaced a file somewhere. All good then.