I played through your game a couple of times. It is a really fun expierience, but I still have some things that I noticed that could be improved upon :)
First up, when you are in high-object count areas (e.g. first boss spam attack, or later almost every room when you max out notes with barrage/Gatling gun) the audio dies and you get a massive framerate drop. I ended up muting the tab, but the audio glitches actually persisted through to the music I had opened in another tab.
Second, you can snipe the first boss from the entrance. You can kill all of his surrounding stuff, but you don't actually take any of his health if I see it right.
Third, I managed to clip consistently into the wall on one occasion, it was the left wall in the bottom left corner of a world 1 room. I tried going into another room while some enemies were still alive, and got offscreen killed by something.
Another thing are the sniping enemies in world 3. I really like the concept of them, but having 3 of them in the same room as a miniboss is a bit much.
On the topic of unbalanced enemies: Green big squares that have the same speed of you can corner you easily, and you can die from a forced hit that feels a bit unfair. Also, purple swarms in world 5/8/etc. are annoying because you can barely see the split up enemies.
Lastly, I second the opinion of bad RNG. The game depends at least for me heavily on when I get the heavy bullet powerup and if I get laser, spin shot or turret. Getting past world 3 with riot shield, bazooka or double trouble is near impossible due to the low damage output. Also, I left world 1 with any number of notes between 7 and 12. Having a meta progression like keeping one power or leveling your stats up from the getgo would be awesome to combat this a bit.
Still really beautiful and awesome game tho :)