I was just messing around with the game and noticed something interesting: pressing opposite directions will cause your projectiles to stay in place. It has limited apllications, yes, but it could be useful against some bosses for maximizing the damage dealt with each shot. Something to consider.
Edit: In fact, I've put together
this movie of the first boss employing this glitch, using your current movie as a basis in order to ensure the exact same conditions. The battle is 114 frames faster (1.9 seconds) than yours as of everything disappearing. Further optmizations could be made to widen the gap: finding the best spot to "plant" the attack, maybe manipulating luck to keep him grounded, etc.
It would be much more difficult to apply this technique to future bosses because of health issues (the fourth especially), but it could still be done. Just plant the attack right in front of him.
That being said, any improvements made will rely on the speed scrolls. In fact, I believe the speed peaks out after a while (I need to test this), and if this is true, you could grab a scroll before your revert back to normal speed to better maintain it.