cute game, I had fun with it for an hour or so.
a few things I noticed:
-I found no reason not to just farm level 1. beating levels didn't feel rewarding on its own (aside from having progressed towards a potential milestone) because the capacity to make more coins/xp didn't go up at all, but the max health of balls went up, essentially making each level up feel slower, making the clear path to stay on level 1 to grind upgrades. if you got more money or other things in higher levels, for example, that could solve that issue.
-spawn time's value is exponential while its cost doesn't go up much per level, so I maxed that out early, which allows you to go to level 1, and spam the -1 level button while your turret instantly shreds the balls, letting you spawn like 20 balls per second.
-I found it frustrating how often the balls would get launched quickly towards the ground after I unlocked splash damage.
-the ufo is cute, and not a bad challenge in a vaccuum, but up until that point you're super mindlessly just shooting balls and enjoying the steady progress, so suddenly having the challenge of actually having to push the grenades back into the UFO is jarring and a little frustrating. also, slow on hit seemed to make it really hard to push grenades towards the ufo.
overall, nice game! I can imagine it being worth expanding on, but it stands on its own as a good time that doesn't overstay its welcome as it is.