Good boss design is difficult, but you managed to do it great! Their patterns are interesting and it's easy to telegraph their movements.

However, I think it got a bit tiresome shooting the bosses after a while. I'm not sure why that is, if it's the lack of music or if there's something else that needs to be added to the game.

i love myself a good boss or two, and i love boss rush modes

This is really cool - I like the way that attacks are telegraphed, it was very clear. Each boss felt varied and unique. The only thing I'd want to develop is maybe making it clearer with feedback when you or the bosses are taking damage (maybe unique sfx for that?) and I feel like it's crying out for a pumping soundtrack to drive the action. Well done!