A bullet hell? Nice!
Alright, to begin with I have to confess one thing: I never defeated the first boss. For a bullet hell game, the player's own hitbox simply seemed too large and acceleration and deceleration on the player character removed the precise movement that is needed in this sort of game.
I also noted that after releasing a charged shot firing a normal one would turn any charged shots on screen back into the standard hearts, greatly reducing the damage that the boss would take from the attack. And with so much happening on screen, I was unlikely to notice when it was safe to fire another shot in order to prevent this.
In terms of a difficulty scale, the first boss has too much health considering the small damage that your regular shot does. It sounds drastic indeed, but I think you could do with halving the bosses health and therefore allow more players to experience the rest of the game, where you can slowly increase the difficulty. You could think of it like a touhou game; even if the player is on the verge of losing the game, most people should at least be able to finish stage 1.
But on the positive side, the music was really good and got me pumped for some rain-dodging action. I also really liked the concept of the game, and it almost feels like a lost NES or GBC title due to the character design, which was great. The concept of a more powerful attack which requires charging was something I had yet to experience in a bullet hell, so it was nice to finally see how that plays out.
If these problems are addressed, then I think this looks like a very promising game. Nice work!