Hi! I have a few Game Gear units and have had my first for over 20 years now. The thing is I have a 53-in-1 cartridge...

Well my first guess would be that you might have used too large capacitors? They would take a longer time to get charge, would run on a different charge cycle (possible flickering) and would keep power up for longer than intended.

I would guess this is also a capacitor issue, or possibly a bad LCD screen. You should try it out with other games as well of course. Capacitors go bad over time and they affect screen and sound and needs to be replaced. You can find good guides now if you google it, does require solvering thought.

It is likely a screen capacitor problem, you can google it to find out more. It could possibly also be a lighting problem if the backlight seems very weak.