Wait this is the coolest take I've seen in SO long for tower defense / vampire survivors / rhythm games! I love how the cost to obtain upgrades is your own level time because it rewards players who go slow and steady (they spend less) AND players who go fast and spend lots.
I feel like a neat improvement to that mechanic could be giving players a little additional seconds on the clock per round (maybe like +1 minute per round) or something; that way it would incentivize spending more because players who clear levels fast enough will farm extra currency. Just a thought. Overall fantastic concept!! And I love the fidget spinner enemies lol