I just bought the game and already put an unhealthy amount of time and focus into it. I read through some of the earlier comments and I can't believe this is your first game! Amazing work!
There have been a lot of good recommendations and compliments in the comments so far so I don't have a ton to add, but I don't think anyone mentioned the idea of bosses. With a wave-based game like this it's perfectly fine without bosses, but it can add a sense of buildup and progression, as well as a fifth enemy for stages. It could be neat if they would be manually triggered by the player either by a large cash sacrifice or by doing certain things like the statue in the jungle stage or like how Jackies appear when you open the backroom of the F.E.R., or just spawn every certain number of waves. I would say that killing a boss gives an alternative way for players to end a level, and beating a boss would give the option to exit or continue and it can track how early the player beat the boss or how many times they beat the boss in a single run, stuff like that.
To be frank though, the current variety is already great. The F.E.R. stage really blows it out of the water with what the enemies do. Can't wait to see what you do next!