Great spritework here and a solid base for a hori danmaku.
When I died while holding slow-mo it continued throughout the first cutscene until I returned to gameplay, making a very slow unskippable cutscene even slower.
Had some visual clarity issues my first few runs where some bullets and the powerups blended into the background. I'd also like to see what hitbox the player has. Some sprites clip through/block part of the hud as well.
The tanky enemies combined with losing all power for getting hit once made the boss fights a slog. It would be cool if there was a way to reclaim some of that lost power. The Touhou games accomplish this by having your power explode out of you as you get hit and you have to collect as much as you can across the screen without getting hit before it falls off screen, that could be a way to handle it aside from rebalancing health values.
The weapon upgrades are mostly good though I don't like how Multi loses its spread properties even though I like the punch ghost, maybe that power up makes more sense for neutral?
The 2nd boss has much more interesting patterns to avoid than the first but the idea of its moveset changing depending on which body part is focused would be pretty dope if there were more oppressive attacks coming from some of them, especially the 3 shot arm cannon.
You could pull a lot of inspiration for bullet patterns from other hori shooters like Salamander, Darius, or Deathsmiles while keeping in mind bullet density to not make it too crazy. Good luck with the development.