Fun game! I loved that you included two different visual styles, almost like you had a "remastered" version at the time of launch :p
My one suggestion is changing how the levels scale. Initially, I had three waves of two blue aliens in a row. I wasn't actually sure if I was advancing levels. Subsequent replays showed that these were random waves. Maybe just have hand-crafted waves? Sure, that means you couldn't have infinite levels, but I think it'll make for a better experience overall.