Pretty solid game here. Having played Realm of the Mad God for the past few months, I felt right at home with the bullet hell mechanics, though obviously there's a decent amount of other stuff to talk about.
I wasn't a fan of the upgrade mechanics. It introduces too much power creep to the game, to the point where there's really no point worrying about your rank as you can S rank the levels with better upgrades later on. Introducing a money multiplier based on your rank would be nice, as it would give incentive for players to get higher rankings earlier in the game, as well as add more options for grinding money other than repeatedly failing the hardest level they've unlocked.
There are a LOT of enemy types here, and while the variety is pretty cool, it seems like there's a lot of potential for mixing different types that you didn't use at all. In most cases, dodging becomes very easy if you just get far enough away from the enemies. This creates a pretty redundant pattern of clearing a corner or side of the arena and then wiggling around in that area while clearing the rest of the room. It works, but there aren't many (if any) enemies that really punish this strategy or force other types of play. The other side of this is how enemies may spawn en masse before you're done clearing a wave. This is interesting and can keep the momentum going, but in some cases (particularly when there are still plenty of enemies left) it can feel really unfair, as the act of clearing enemies ultimately results in more appearing on screen. It can feel like the player is being punished for doing nothing wrong (it's also annoying when you rush in to grab some coins only to have to retreat immediately.) It would be more fair for only weaker enemies to spawn in these kinds of situations, and if your intent is to prevent players from camping in "cold spots" this could be done by a certain enemy type rather than through the game's spawning mechanic. You could also do this with more than just bullets; obstacles such as walls or traps (Amorphous uses puddles of things like oil or acid) would be a creative way to increase the skill ceiling of the game without just taking a "more is better" approach to the bullets.
Performance-wise, it honestly looks like you did the worst thing you possibly could. The number of objects moving on screen for a bullet hell is higher than potentially any other genre, and you did this while also putting glow effects on each of the bullets (since filters don't work in Ruffle, I'm guessing you used Soften Fill Edges?) I had to download this game and run it in Flash, and even then the game slowed down considerably. I switched the game quality to low, which worked for a few levels... and then, even on Flash Player and on low quality, I still experienced some significant slowdowns. Surely this game would run faster if your glow effects were pre-baked with PNGs?
Aesthetics-wise, the graphics look nice but it's obviously unfortunate that you had to re-use old assets. I thoroughly resented the music, but I'll admit that's a matter of taste.
Good game and engaging gameplay, but the content feels a bit padded.