A lot of the difficulty variables like spawn rate, speed multiplier, etc. max out at wave 20. After that it's only the number of enemies per wave that continue to increase, so by that point you've pretty much "beat" the game. We had very little time in the end to do any actual balancing so a lot of the values in the game (like money earned per wave) are a bit crazy.
I really appreciate the info on how far you made it, it will be very helpful incase we decide to do a little balancing post-jam.
Thanks for playing and for the feedback! :)