I finally got back to this and finished the game! The difficulty definitely ramped up with the final few levels, and the final boss was very well done. This is a very fun experience that I would be happy to recommend to others who enjoy these sorts of games. I did run into just a few issues that I didn't catch in my last run, so here they are, if you're interested:
- Level 1-2 has three chests but only displays two in the info box (opening both chests at the same time seems to count as one, because the right chest was back to being unopened when I returned to this level later).
- Level 1-5 also has three chests but only displays two in the info box.
- The bloody rightward arrow in the bottom path of 1-5 has collision as if it was a solid object.
- I wasn't able to find any chests in 3-2. Maybe they're just really well-hidden?
- I'm not sure if this is unintended, but getting one of the yellow stones for the mana booster upgrade and dying in the level causes it to respawn but still add to your total (I happened to notice that the one I picked up in the first world didn't respawn when I revisited the level).