Very cool game. I can tell you crunched some numbers in order to balance the progression and difficulty. Left me on a tiny grind in order to buy the 9k upgrades but I never felt the game is unfair.
Some technical feedback: I got to fight the second boss pretty early, before buying any major green upgrades for second phase and for a long time I was left with forced game over whenever boss reached half of his life - it wasn't clear what should I upgrade in order to be able to kill the boss, or if I needed to upgrade anything at all.
Also, I encountered few random lags on a desktop version + sometimes I think I saw the upgrade screen popup for a split second couple of times, when I clicked and dragged mouse in the area of the top bar. The game is probably trying to drag the update screen - you might want to disable it during the time arena, or put it in a different scene if you haven't done it already.