Very good game, nice mechanics and well made difficulty curve, I had fun playing it and will come back to it.
The boss mechanics and so on makes me think of enter the gungeon and modded terraria levels of AI, they are hard but mostly predictable definitely the best in my opinion for this kind of game!
I noticed the red bullets sometimes dealt 2 damage, sometimes 1, if they are different bullet based on enemy I'd make them 2 different colours, if it is a double-collision issue I'm fine with it, doesn't make the game unplayable and the time contraint don't make it easy to fix this last minute.