Thanks for commenting and taking the time to play through a hole loop :)! Yeah the hitbox of the spikes is a problem, I didn't know how to manually adjust them (they're used in a tilemap) so I went with the default hitbox, which is, as you pointed out, bad on edges. I was also surprised when I watched some other people play it, that the platforming was harder than I thought. Definitely something I take with me, that some jumps that may seem easy for me (as I'm used to how the player jumps, moves etc.) aren't for someone thats playing for the first time. Also shorten the levels is good advice!
Oh, and the thing you missed was a gravity cave where you could change gravity from +1 to -1 with different switches.