I see what you're going for and i love it, i had #DakkaDakkaMech as one of my 2D game ideas and i'm glad you're doing it.
The fact that the bullets are stacking is fantastic, midway through the boss battle i fell down expecting to be at the start of the boss area but instead i landed softly on my own bullet casings which had filled all the way up to the platforms.
Only critique i can come up with is that the shooting does not feel all that impactful, feels like you're shooting the enemies with a BB gun. Although I'm late trying out your game and I've seen you progress since then.
Good work, keep it up!