I love the visuals, they look super pretty. I also really like the heart mechanic - it forces you to be on the move so you can't just stay in one spot all the time. The bomb effect is also really pretty and satisfying when you get it.
I'd add more lives, because the game gets really fun later on but it sucks getting hit once then starting over and having to go through the really easy waves every time. I would also add in some visual indicator for where the bullets are about to spawn, because it's easy to get blindsided by a fast-moving bullet that spawned near you.
Overall, simple but very fun game. Great job!