Make the upgrades get better as time goes on! Going up plus one damage at wave 6 isn’t too ideal. You could also add visuals to the upgrades, such as changing the weapon the more damage you do.
i liked the game! however the wave 6 boss from hard mode is very very difficult due to robots being on top of the boss (i was able to reduce the health to 6 hp :')), also in between rounds the bullets shot from the robots aren't disappearing