I like the concept. basically Tower Defense and Vampire Survivor combined. And also a high score system to make it worth the grind. I had fun trying to reach the desired average playtime u wanted (but failed).
I am not sure how much it would affect gameplay but making the camera FOV a bit bigger would be nice, too actually see whats in front when the gunner started firing.
Currently, since everything is basically sprites on a mostly solid color background, things seems kinda flat, would be nice if u can add shadows.
Balancing wise, I dont have much experience, but currently, either enemies grow too fast, the power up too week, the power up frequency too low, RNG sometimes just harsh. Maybe you can somehow convert these things into some value point and use them to decide the stats of enemies and power ups popping up.