This game has a real core issue of a "lack of decision-making." The premise is simple and quite charming, but it suffers from this fact: It's a pretty mind numbing game. The core of a shmup is fast-paced decision making, constantly juggling whether to close in, move away, whether to leave some enemies alive for a higher score, managing resources etc... In this game, every wave is very segmented and there's no way to "fail" a wave. (In most shmups, waves have a timer after which they leave so you can't farm them infinitely for points). You either clear it or it goes on forever until you do, and there are no resources besides your lives (I frankly still have no clue what "Core Stability" is I never saw the big crystals on my right deplete.), so the only thing you have to do is focus on micrododging the patterns and out-DPSing the enemies (for this reason the Armband Memento is always the superior choice because it lets you accumulate damage without having to look at the enemies and you can just spam the homing shots by mashing X while holding Z to do a barrage of both shot types).
The boss fights also suffer from this a fair amount; I was expecting them to move around the screen in ways that would, say, give the Pendant Memento room to shoot them from behind or from the sides, but instead they sit at the top of the screen and fire patterns that can really only be dodged in one way consistently and then you just... outDPS them. The one time one of them moved down in a way that would benefit the Pendant was the first boss with his attack where he charges down... And then he suddenly comes from the top of the screen for no reason and cheapshots you if you hang out there (in general the game punishes you a LOT for being at the top of the screen with crystals coming down superfast and immediately killing you-- Do they really need contact damage?)
The fish actually is pretty fun to use (and it's immensely charming) but then that leads into the smaller issues with the gamefeel like the fact that even though it's super satisfying to shatter the crystals, the most common enemy type isn't the small one but the big one that has multiple HP so even your super cool fish swing has to swing twice to shatter them (and there's no sound effects for doing damage) so it feels weak. Also, since a lot of patterns don't have "cooldowns" (where they stop firing), a lot of them are just "kill a bunch of crystals at the start to make it bearable or else they will pelt you with an undodgeable barrage of shots the moment they get going."
When I first started this and saw the crystals teleport one by one and I saw the background synced to the music beat, I thought it'd be more of a rhythm game. I think if it was more like Space Invaders Extreme 2 where everything is synced to the music (i.e your shots ring in tune with the melody and enemy spawns and such can be all synced) then with some added visual effects I think you have a really cool "trascendental experience" sort of game like Tetris Effect, and you add _something_ for the player to think about besides just micrododging. I think it would work really well with the crystal aesthetic and the music (which are both VERY cool); either way this game seriously needs more meat on its bones. Make the player think more!!