First thing you can try is cleaning the contacts in the charging/audio port with some IPA. If that doesn't help, my next guess would be a faulty solder joint
If you have a multimeter on hand, you could measure the current draw from the battery, that way you can estimate battery life and compare it to other flashcarts.
Definitely replace the CP1 cap. Most OG GBA volume issues I've seen were also simply related to bad caps, especially if the volume slider/wheel is good
Owned two Omega DEs, both the old one and my current one were flawless the entire time and I haven't heard much about others having issues at all. The DE is leagues ahead of the standard Omega in just about every way.
Yup, I've heard that tons of 90s PCs are starting to have cap leaks as well. They are timebombs and the liquid electrolyte inside is really corrosive. Swapping these is a nobrainer imo.
FF6 Advance is arguably the best version of FF6 and LoZ AlttP GBA has some decent QoL improvements and additional content over the original. I think all GBA ports have a common disadvantage though: worse sound
Not an expert but curious as well! I'd ask myself what component is responsible for the brightness switching in the first place. Maybe it's just a faulty voltage regulator somewhere?