An intriguing entry. I liked the weapon customization system. The UI took me a little while to figure out, but once I did it was fun to try out some different options.
My biggest complaint was that the enemy bullets moved too fast, resulting in a lot of constant circle-strafing. This type of gameplay is okay in small doses, but it's less interesting than the dodging-and-weaving around slower bullets at the heart of most bullet hells. Remember that slow-moving bullets stay on the screen longer, and as a result can actually present a greater challenge than bullets that move quickly. This also gives the player more agency to select a path of motion--a win/win most of the time.
I also wish the sounds and music were not so noisy and repetitive. Sounds that play often, like an enemy's shooting sound, need to be quieter and more subtle than this.
Good job on completing your game, and making so much content in a short period. Nine bosses is a lot!