I love where this is going! You have a great foundation with the prototype for a larger release. The idea of skill trees and upgrades is fitting in this stadium-type creature collector. The auto-attack setting it neat, providing a sort of rhythmic undercurrent to the game.
I do recommend focusing some on explaining controls and conventions. It took me a little bit of time to figure out how to play, but once I did it really was a fun experience!
I look forward to seeing this project grow into its own proper, full-fledged release. The DIY hand-made aesthetic is refreshing in a sea of AI.