I'm going to start by saying I love this game; Unfortunately I can't rate it because I didn't join this jam this time around. I do have a lot of thoughts about it though

Great Art: It really is a fun game to look at. And there isn't too much going on which is nice when there's that many enemies and xp on screen.

I love the upgrades/downgrades: In particular the downgrades were well thought out to balance the upgrades in various interesting ways, and promoted buying a lot of different upgrades instead of just the best one every time you see it. This is great because it makes it harder to metagame; there's not always a clear best choice even though the upgrades are simple and easy to understand. I almost bought something that would have killed me one time which was an interesting experience. The only thing about them is that I want more of them, It's a three day game jam game but if you want to develop it further I would add more of these because the more upgrades and interactions there are between them the more replayable the game is. (there were no upgrades that increased health and reduces damage or attack speed for example)

Lag: I've seen comments about lag. I never experienced it in a 21 minute long game, but if people are having lag you might want to make the XP clumps have more range to draw each other in. Or make sure that the logic for XP isn't running all the time. If each XP is running code to check for other XP every frame it might cause more harm than good. If so make the XP pickups only check for others to clump up with the moment they appear.

Also I've noticed that XP is often hard to pick up right away which is fine, but consider having them drop red XP orbs or something from time to time that lets you pickup all XP that's laying around like in vampire survivors. You could even make it so that dark blue clumps turn into red orbs if they clump up. (for example 5 light blues make a dark blue and 5 dark blues make a red.) the numbers are something you would have to experiment with.

The flies: There was only one enemy in the game but I like how they had an attack range instead of needing to touch the player directly to attack; This means you need to keep your distance from them in all directions.

I have a couple of suggestions for enemies.

Bees: if you draw a few yellow stripes on the fly and maybe add red eyes you have a solid bee sprite so that would be an easy way to add more variety in enemies. I was thinking the bees could have more damage, range and hp but overshoot you instead of going back to where they were and you can dodge their charge if you have enough movement speed. I was also thinking that if you make them spawn in late game instead of flies you could add beehives earlier on that your player will auto aim at sometimes and destroy if you stay too close for too long which will cause some amount of bees to spawn in it's place.

Frogs: They will leap to places near you but not within their attack range, If you step too close to them they attack you with their tongue drawing you even closer to them and dealing a lot of damage.

In conclusion: This is a great game especially for a 3 day game jam project and you could definitely sell it on steam or here if you add some more enemies and upgrade/downgrades. At least I would pay at least $15 for it if you did maybe even more.