Nice work on the tank and scorpion models - and the death animation was a nice touch.
Optimization wise there's a bit of a lag when a bunch of enemies die at once, I'm wondering if you're not already using object pooling for the scorpions if that would improve that performance a bit (rather than destroying enemies, just hide them and switch their position to off screen until you need them again for a respawn).
Design wise I think I have to agree that the invisibility seems a little underused in this game, as besides at the very beginning it doesn't seem to help much. I wonder if there could be unlocks that could pair with it to make it more valid (eg; phasing - where you can pass through enemies when invisible, plasma shells - where you can still fire when invisible). Also it seems like maybe more iterations on balance (slow down initial enemy growth waves, rate of fire & AOE seem to ramp up really fast to be the solution to most problems, etc...)