Very kind of you to say, thank you! And I really appreciate that you took the time to write such an in-depth comment!
I agree that I could've been a bit better with communicating your options for upgrading/selling, but this being a game jam I simply was hoping people would stumble upon it because I just didn't have time left for a decent tutorial.
The current strategic element is mainly to combine high fire-rate, low damage towers with the inverse of those and with that create a mix that is able to kill both small and larger units. But I agree with you, a slow down tower or any other similar towers with a more synergistic element to it would be good to add.
I also have an idea of how to make it more clear how much money you actually gained from this wave and how to offset how much it affects the difficulty curve. So I'm glad to hear that you pointed that out.
Thanks again for the nice comment and thanks for playing! Have a great day!