I played some more and here's what I think:
Im getting overwhelmed a bit with more enemies now
an idea I had to fix this easily could be a speed-up button. You can just increase the game's fps. Id make parts of the game go faster to
Or you can also give wave complete bonuses
I also had a negative lives and couldn't loose
I think you should add a really beefy enemy in the game more earlier on since I could spam splash towers easily
If you'd like you can try out my game dot TD hardly anyone plays it so its whatever. If you are gonna play it you can press space to speed up the game since no one plays it.