I really appreciate your detailed review!
If I continue building the project the music is definitely something I'd improve since for longer runs it gets kind of repetitive.
The intention with the fireballs was to have them grow until full size and then stay that size but maybe I should have them start larger so they're easier to see.
I agree about the health being confusing, the intention was that it would give you enough extra health that it blocks an additional hit before you die for each one you pick up. I think the reason the health power up may be bugged because dying removes a stack of health but getting hit also removes a stack. Without some kind of life bar it's hard to tell when you can take another hit.
I definitely would want to add more animations and an even larger boss style enemy that's designed to be more challenging once you have enough power ups and get past the initial difficulty curve.