Playing this pretty late! As others have said, the graphics and audio are nice, the story just enough to fit the theme.
Some feedback per category:
Performance: I had an issue with my monitor flickering in fullscreen, even after making sure refresh rate was 60Hz not 59Hz. But an alt+enter to toggle windowed mode, and it was alright, I could play in Low Quality mode.
Progression: difficult to train as there is no menu to start from any game phase. In particular, there was no way to skip the tutorial so redoing everything until you reached the phase you haven’t learned yet took some time (same issue with most shmups with no level/section selection). Fortunately, the Story mode can be used to reach any point in the game easily (but you still need to take some time). The last part is probably easier than the second one because the hitbox of the enemy boat is huge, so you can sometimes stay under cover and still hit. With the exception of the tidal waves (probably a little too frequent, in fact) that requires you to jump very high. The thunderstorms were interesting, it seems the only way to dodge them was to jump aside and come back (roll can help).
I was positively surprised by the number of phases and parts, I thought it would stop after the 1st part! In fact, in the difficulty settings, besides changing initial protagonist HP, I think you could add an option to change the damage per seconds of the main attack. Because each phase is pretty long already, but there are many phases and 3 parts, so I think it you have enough material and variety to keep the player’s attention. Rather, changing phase often, as in Touhou bosses, would keep the player’s attention even more. And players who play more aggressively (hiding less behind the cabin and aiming continuously at the boss) may also feel rewarded. Combine this with either more initial HP or HP recovery between parts, and you can have an intense experience where, in fact, you decreased difficulty, but the player will still be under pressure because the bullet patterns are the same and they can still die, but it’s up to them to get out of cover and deal more damage to the boss quickly to finish the phase.
Finally, Sin & Punishment-style, you could add an option to enable checkpoint for every part and/or boss phase, for a hybrid between pure Store mode and still requiring the player to master a certain phase before proceeding.
Gameplay: I like the concept of 3D bullet hell, but combined with precise shooting (except for part 3) it made it quite difficult. The boss hurtbox seemed more generous than I thought, so I gave up on always right-button hold aiming which prevented me from seeing around and put me too much in danger, only aiming from far and maybe occasinally recalibrating with right-button hold for a short time.
I’m used to Sin & Punishment gameplay where bullets are coming from the front, and a classic left/move + jump is enough to dodge most bullets. Sin & Punishment 2 adds flying so vertical motion, but you stay in place in the air and bullets still coming from the front + special bullets and lasers covering the screen in 2D patterns, so it’s all manageable as you don’t have to rotate the camera, basically.
This game and also other shmups also feature many intermediate enemies that act as small rewards. In this case, the other targets are the ghost ships, that you have to destroy to protect yourself (it seems that a ship crash causes you 1 damage, and in the whirlpool, some ships don’t crash but may touch you), but you don’t “gain” anything except surviving. In shmups, it’s less a problem even if small enemies don’t give much score or power-up because your straight fire will probably hit other units anyway, so it’s all in one go, but here, you have to switch to weapon 2 and hit just one specific ship, which means you hit no other ships and most importantly, you stop dealing damage to the boss. After realizing that I was spending my time destroying ships for nothing in the whirlpool, I switched to just attacking the boss, but if I wasn’t in Story mode, the many ships would kill me in no time. Maybe if destroying a ship was consuming some of the boss’s health, it would be more beneficial. Generally speaking, I think there are too many ships in the whirlpool section, unless I misunderstood and some of them were not hitting you but just for show.
I do understand why there are not a bunch of small enemies though, it would be even harder to hit them all in 3D while also hitting the boss (again, unless they all consume the boss HP), so having just bullets and big enemies may be better in a format where the boss is always present (if there are only minions as in Sin & Punishment, or that MechJam III game CyberLancer, that would be feasible).
The roll goes a bit far and the character doesn’t blink so I’m not sure if it provides invincibility frames or if it’s just a move to evade.
Characters: the familiar is fun, but I didn’t notice it during the game, only in the last scene, I suppose it was disturbing the player?