Thanks very much for the review! This is great advice. A lot of the things you've noticed are things I have been working to fix in the later development of the game. (V1.1 has some fixes for some of the things you have noticed, and I'm still working on a 'final' release with a fully triggered ending and such).
Your response to the howl + down to collect is really helpful; you're the second player to make this comment and in practice, I was trying something that doesn't seem to have panned out. Might not make a big difference for this game is it is right now, but it'll inform the things I try when developing future games.
The GBJam release doesn't have all of the music implemented properly, so you are correct about the music feeling more limited than intended. The note about more gimmicks is interesting; I will definitely keep that in mind for improving level diversity in feel!
I don't think this was over-critical or anything, I appreciate the helpful feedback and notes to improve on. This is my first game-jam game ever, and I was working solo so every piece of advice is a huge help! Thanks again!