This was addictive. Like I had to stop myself.
My singular note is that the progression seems static in terms of which ones spawn. Ie, the spawn tables seem to never change weighting imo. It would be nice if at the end you saw mostly the Three Head, Hooded Lurker, etc. (sorry if I'm getting the names wrong!) since by the time you get kitted out, the beginner ones are trivial! That's it.
If this had existed when I was in middle school there is no saying how much time I would have spent on it. It's got that correct blend of chill and active. You designed a very good core mechanic and upgrade system.
Proof of how much I played:
Great job :)