Amazing work, congrats! Great music and graphics, the whole package works nice. I love the procedural backgrounds also. I beat the first boss, my high score was 32700
My feedback...
There are a few simple changes to the UI that would greatly increase screen space. Move the score, hearts, and special to the bottom of the screen to improve visibility of incoming enemies. Instead of showing each heart just do like x3 to save space. Also, the score always ends in 0, it's kind of a waste. You could knock it down to 3 digits.
The levels are a bit too long, maybe if there was a progress meter it would help?
The spread shot feels overpowered.
The player's hit box could be much smaller, like almost a point.
The bullets are a bit hard to see, they could be brighter to stand out.
The special should destroy enemy projectiles.
I hope that helps!