So I did manage to solve this problem by following the guide over at this wonderful place. It doesn’t pertain specifically to MGBA, but rather Gambatte, but both do a solid job with GB and GBC games.
Gambatte-libretro aspect error with lima driver in v0.3 OS
It took me a fair share of tinkering, but if you need some additional help, let me know. I’d be happy to assist as I can.