I think it's a fun concept! The bloom on the main menu is quite aggressive but on the game it's fine. The shop mechanic is quite interesting too! Though I would want more gameplay elements that uses the countdown theme more extensively beside a countdown timer. I like that the timer is your health though!
The boss is pretty difficult to deal with since there are other enemies that I need to focus on too. Overall it's quite well made, I think some more sound effects for player input could be nice, for example a shooting sound, and maybe some moving background could work!