Nice game! I liked the main mechanic (pick the bullets up or not); a small QoL improvement would be to autocollect when the room is empty. The MC is cute, and the story is too :)
For my taste is wasn't enough of a "bullet hell" until the last boss came up and more of a classic slasher/shoot-em-up game. The shrinking theme is well covered!
I think forcing the player to abandon the bullets more to be able to move faster/dodge would be an interesting direction. My strategy was mostly "shoot, pick em up while dodging, shoot some more".
I had some minor graphical/music glitches, but I played on Linux in a VM, so there is a lot that could go wrong there :D