Omg thanks a lot for all this valuable feedback! We will see how we feel about the game some time from now and decide where to go from there.
Camera zooming out is a nice way of introducing some sort of prestige system!
As for the last few upgrades being too expensive and not very impactful, that's true and it's the reason you can access the boss stage early. If you need help with it, you can grind out more XP for those upgrades, from the normal stages or from the boss stage itself. Of course if we were to expand the game most upgrades would be reworked and rebalanced