All the levels before what's at the very top of the main map don't use any meta or hidden mechanics, though the extras could be a difficulty spike.
Viewing post in Baba Is You Levelpacks: Savefile Swamps comments
Sorry, I had forgotten that level 0 on the right does use a trick that you might not have encountered before, which makes it significantly harder than the opening levels on the left and maybe on par with the extra levels on the left. The right-side versions are intended to be more difficult than the left-side versions.
I understand how... not necessarily bad, but still not good (please let that make sense) of an idea it is to give hints to your own game/levels/etc., but do the 5 levels I'm stuck on have anything to do with savefiles, (separate) levelpacks, or neither and It only requires one save file and it's possible without the other levelpack.