Pretty fun! I'll say as friendly critique, having both in-run and out-of-run upgrades be unit specific, plus not having too many synergy abilities between units, really makes it optimal to silo all upgrades into the first two basic units.
I was struggling on the second/third boss until I figured this out as well. Putting some into healer helped too, because it gives major damage boosts on heal. Other units simply weren't worth upgrading or even purchasing.
