Thanks for playing! I am considering where to take this game from here with roguelike upgrades or waves. The difficulty was definitely something I tried to fix, but just ran out of time :/.
I think a system where you have either timed waves or waves based on enemy numbers would work, then when a wave is finished give the player a choice between 2 or 3 upgrades, upgrades could have certain strengths against some enemies, and after that gradually increase the difficulty of each wave. this is the first solution that comes to my mind, implement your unique twists as you see fit :)