Really cool concept ! The tradeoff is really smart, I liked it ! I found the upgrades to be quite expensive early, it takes a lot of runs (I did get blasted a lot on the second wave) to unlock a single upgrade. Since those are not really impacting individually, maybe lowering the base price and increasing it after each upgrade could help a lot in the early game without being overkill ?
Cheers !I had a lot of fun :)