Congratulations on your first game! Aside from what others have pointed out, I'd also suggest some more dynamic enemy design. Maybe some enemies that speed up others around it, have erratic movement, shoot projectiles that can be destroyed by your shots, etc. These are just examples, not direct suggestions.
And the upgrades seemed almost entirely in favor of enemy weakness, as it essentially acts as +damage most of the time. There's no reason to pick HP because you simply outdamage enemies until you get outdamaged. HP doesn't change the outcome when they all gang up on you, it's already inevitable by that point. Clip size is alright at first, but after a few upgrades, it increases your overall damage output by a negligible amount compared to weakening enemies directly. (Edit: I now realize I probably just got unlucky and weapon upgrade improves a random aspect of your weapon. Is this correct?)
Also, the spinning should be more important to your game, but it's understandable because this is your first jam. It's a learning experience!
I like the idea of a tower defense where you have to aim though, just needs some more mechanical depth and balancing!