As a fellow first-time jammer, I'd say that the difficulty curve is somewhat reasonable, albeit a little flat. An admittedly large portion of the earlier levels felt like they basically played themselves, though I can see this becoming less of a problem as development continues. The final level was definitely the high point of the game for me, mainly because of how it took familiar mechanics (namely the "exit level" card and the inverted S card) and explored them further. I would love to see some of the other mechanics fleshed out in much the same way, specifically the reversed jump card and the normal S card. I found a glitch involving the normal S card that allowed me to clip through regular floors, but I could imagine that being turned into an actual game mechanic with some mild tweaking ;) I also wonder if maybe placing a limit on the number of times certain mechanics can be used in some levels might allow for more interesting puzzles. I hope this helps, and I wish you good luck on your game development journey!