After playing through all the games again to do a proper voting, I realized some tiny flaws in your game.
When running on a classic Game Boy (DMG) you will have some minor slowdown issues while shooting, if there is also an enemy on screen.
The other thing is that momentum is kept after dying. I also had this issue in my last game, it kept momentum after teleporting.
I sadly can't really remember how I solved this. :D
*Btw, after giving this a few playthroughs, I don't think this game is hard anymore. :)