Hi! I really liked the game, it was fun! The boss was also greatly made, but i have some suggestions:
1) You should add invisibilty frames on dash and/or after getting hit.
2) Maybe increase cooldown on those machine gun enemies? They are cool, but nonstoping "rays" of bullets are tough xd
3) I had a bug where an enemy spawned inside my character so i got stuck. Maybe before spawning an enemy there should be an indicator of some sort.
4) Simple highlight on enemy bullets. On a big screen the bullets might be difficult to see.
5) You should add more variety of buffs that decrease size. And overall balance of buffs. It's now more efficient to take health, speed and dash buffs (which decrease size) than some powerups. In my run i took only 2 red buffs - the "more bullets" one and 1 "fire rate" - everything else was blue. Maybe some buffs should be in different categories? For example "+max health" in red?
All in all, I was happy to play your game, Well done!