This is a really, really good entry for the jam limitation. Having infinite ammo meant I got myself into trouble very frequently, but I always knew it was my fault so I didn't mind at all. To optimize time, I found that as the board dwindled to just a small group of blocks I would spam bullets at them and hope I won before they rebounded, which had that perfect feeling of high risk high reward. Really fun!
I have some small suggestions that I think would help enhance the experience for the player:
There were times when a ball would come at me horizontally so it was difficult or impossible to dodge. This was usually because of something I had done anyway, but it did feel a little unfair. Maybe you could add some sort of a "dash" feature that gives I-frames? It wouldn't need to be spammable. Alternatively, a lot of bullet hell games have "bombs" that clear the screen of all bullets and are limited in ammunition. Just some ideas :)