Congrats on making a really fun game! The stealth mechanics were very cool, and the decoy ability was pretty unique! A few things that I want to mention; firstly the player's hitboxes were very undefined. This was especially hard in the boss battle, where the bullet hell style fight was tough (and kinda unfair) without clear hitboxes.
Speaking of the boss... The boss didn't really fit with the rest of the game. The game had been training players to use the stealth mechanics; the dimensional hole things and the decoy ability. The boss didn't make use of holes at all, and the decoy wasn't remotely helpful. The boss felt like a vanilla bullet-hell boss, rather than something unique to your special mechanics. The boss's attacks were also pretty boring, it never did anything interesting. Maybe the boss could turn it's back to you, as it charges a move, giving you an opportunity to slip out of sight and engage in stealth? Maybe you could use the decoy to make the boss shoot something, damaging the boss? Perhaps there could be some moves that can't be avoided at all, and require you to hide in the holes? Maybe there could be a lot of obstacles around the place to let you hide from the boss? Or what if you were to combo these for maximum effect, like you use the decoy to manipulate the boss to shoot some supporting column, which damages the roof, dropping rocks that you can hide behind? I think there's a lot more interesting things that could've been done, hopefully these suggestions help with future bosses.
Otherwise, the game was really great, outstanding visuals and stealth gameplay! I would love to see the game fleshed out!