As pretty much the entire experience was positive, I'll only leave the criticisms here otherwise I'd just explain why I loved every single thing and that would be pretty much pointless as I would describe the whole game.
- The volume keys on my keyboard skipped the intro. I assume you have it so any key skips it, but maybe just leave it to enter or space or something?
- First trip to the shop and I wasn't sure where the money indicator was. You might want to add "G" or something next to it on the top of the screen so it's consistent with the shop. Needless to say I figured it out pretty quick but still a little nitpick.
- In the shop W looks like an H. Super tiny nitpick but worth mentioning.
- The scanlines don't effect the cursor. Maybe this is to easily distinguish it? Still seemed kind of odd aesthetically, however.
- The bosses seemed challenging, fun, and fair except for Behemoth (out of the ones I played). He seemed a bit too unpredictable and hard to follow what's going on, especially when he starts to litter the entire screen with bullets.
- Waterfalls don't slow music like the water areas do.
- Some enemies (like the small brown guys) are hard to follow because they look similar to the bullets.
- Can sometimes be hard to find the player amidst all the chaos but I understand this is typical for bullet hells and difficult to have a solution for so I don't fault you for that.