Very unique and the gameplay loop is really fun, loved the music too. I think with a bit more content and difficulty scaling this can be a banger of a game.
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I think a system where you have either timed waves or waves based on enemy numbers would work, then when a wave is finished give the player a choice between 2 or 3 upgrades, upgrades could have certain strengths against some enemies, and after that gradually increase the difficulty of each wave. this is the first solution that comes to my mind, implement your unique twists as you see fit :)