man, you work fast. most issues i had are already solved. though, i still have some words.
my recommendations depend entirely on the direction you want to take the game. if you just want some fun little incremental to burn half an hour then you are basically there, but if you want something a bit more big or just time sink then:
1. i would recommend other types of swarm units, divers fill attack, bombers fill AOE, but i think it would be fun with a more developed class system such as defensive units that act as a shell around the main swarm to prevent losses, units that either support nearby units or respawn them, stationary area denial, etc. of course many of these only work on a larger map and may lead to a more planning heavy zerg-style game bit i just like that.
2. larger maps in general, the asteroids are a nice start but there is potential for much more in my opinion. depending on if you want to stick to the quick wave action then it could be more maze like or even allow player set up, if you end up going slower more true devouring and replicating zerg feeling then larger generated maps with perhaps point of interest to give buffs or win.
3. as of the current state, it is a 15 minute experience at best, and that's fine if you want it like that. if you want more then i would recommend a variety of enemy behaviors and types/strengths, perhaps having a choice or tree of enemies to pursue fights against to get or develop your own troop types, and just general things to work around or discover in order to keep from the loop of just moving in a circle or just sitting still.
4. i still have little clue what the formation button truly does, and would appreciate an explanation or perhaps tool tip system like the stats when hovering over it.
all in all, still incredible work all around, but there is only so much feedback i can give without knowing the intended plan or scale of the game such as what the particles actually are like a zerg galaxy devouring swarm, nanites, if there will be story, if its just a basic arcade game, etc, as its already a decently rounded small experience as is. my apologies. I'll come back if i can think of a good name to recommend.