I'll say from the outset that bullet-hells/vertical scroll shooters aren't usually my sort of game, so definitely take my feedback here with a grain of salt as far as the genre is concerned.
AESTHETICS
The game looks quite nice! It's simplistic, but with the possible exception of the player's ship, everything on the screen at any given time either matches the look of the drifting space around it or stands out in stark contrast so that the player can see it. I like the fact that the player is almost always the most complex and visually striking thing on the screen, as it gives us a feeling of being special and unique in the game's 'world' while also making it practically impossible to lose sight of yourself. I think the bosses could be made to match that level of quality, to give them that sense of actually being on par with the player as far as complexity and power is concerned, but overall everything looks good for what it is.
SOUND
As was mentioned in Cyberdevil's review, the music is a bit droning. Most things on the screen are moving fairly slowly, and when combined with the steady hum of the background, it makes the game feel even less urgent than it already does. It's be a great accompaniment to something where slow, careful movement was the focus of the game, but on something that's theoretically meant to be faster-paced and action-packed, it doesn't add much to the experience.
The sound effects of the game get the job done - your typical bleeps and bloops and old-school scratching effects for explosions. They don't seem out of place against the game's similarly old-school look, but they aren't particularly noteworthy either.
An important note, but you should probably have the muting of the music and sound be separate features. I hit mute by phase three because I was trying to focus on writing this, but when I went back up to fiddle with some controls for that section of the review, it was quite disorienting to try and play without any sounds for firing, taking damage, etc. If someone found the music distracting or merely wanted to play their own while playing the game, the fact that they can't do so without removing audio feedback as well is something of a game-killer.
GAMEPLAY
From the outset, I think it might be a good idea to bump up the size of the stat block the player is building with their ship choices. I honestly didn't even notice it until it was disappearing as I started the game. I like that I get to choose three ships for different specializations, but it should definitely be much more prominent what those specializations are going to be.
The ship control is really floaty, and while that may be a bad thing for a lot of shmups like this, I was a fan of how 'spacey' it felt. With the lower-control ships, it felt good to thrust in a direction and take a few pot-shots as I drifted along. The only complaint I have about it is that the game's art doesn't really reflect the difference between the two states. A minor dark-to-light gradient that reflected the ship listing to one side as you thrusted and eased off when you were merely drifting would be a nice touch, I think.
The different ships really do handle quite differently, and the slight drift of swapping between them feels slow enough to discourage doing it in the middle of a storm of bullets, while still being fast enough that you're not likely to have players hammering the key, wondering why it hasn't kicked in yet. Solid implementation of the mechanic all around.
Without power-ups, varied hit markers, or more drastic weapon effects, the player's bullets don't reflect differences in power particularly well outside of certain enemies dying more quickly. You know that the Red ship hits harder than the Green ship because it has two bullets per shot to Green's one, but the difference in power between Red and Blue is far harder to determine without spending a wave or two comparing kill speeds. I realize that the limited art style precludes using things like explosions or the like to really hammer home 'this shot hurts more than the other', so this might be a place where taking particular care with the sound design would be most helpful. If Red's shots land with a THUD and Blue's with a more electrical 'blip' or 'snap', players can tell innately which one is hitting harder.
The inclusion of a 'pacifist' mode is actually quite clever for a game like this, I think! Challenging the player to deal with the enemies purely through dodging rather than eliminating the sources of threat seems like a fun take on the usual bullet-hell formula, but in trying to take this path, it revealed a problem: The game is too slow and too simple in the early waves for this path to be rewarding. With the exception of the second boss (which seems to require that you shoot it a few times before it will move on, contrary to any other enemies prior) and the yellow homing square before it, it was more or less possible to drift up to the top corners of the screen and simply wait for each wave to end. With enemies moving as slowly as they do to give the usual player time to kill them, this could get dull pretty quickly. And even on the second boss, which couldn't be killed purely by waiting it out, huge chunks on either side of the screen were never touched by bullets at all. Having more enemies that could track the player's location and shoot directly at them, or enemies that would home in and attempt to impact the player, would do a lot to give even the pacifist players something to do other than putz around at the top of the screen waiting for each wave to end.
CONCLUSION
It's a fun little game with a good look to it, but it starts slowly enough that a few holes in the design peek through every now and again. It's not going to convert anyone who's not already a fan of these vertical scrollers, but it's a decent enough time-waster for a few levels and a solid foundation if it's going to be iterated on in the future.