I have an idea for the collectible notes; maybe when you collect one, the speed or power of the note(s) that it affects increases.
A list of other things I would suggest (according to my experience):
- Add an upgrade that increases how much you see
- Make shops more common (i know others have said this)
- Increase the average amount of "coins" you get from enemy and miniboss rooms
btw, circle burst is probably too op, I beat the first boss with lv. 1 circle burst and 2 full rows of notes in about 15 - 20 seconds