You should download the GBA Balatro homebrew. It’s seriously addictive: https://github.com/GBALATRO/balatro-gba

Also tactical RPGs are great, like Final Fantasy Tactics Advance, Advance Wars 1+2, and any of the Fire Emblem GBA games (look for translated roms).

I also especially like old Lucas arts (and similar) adventure games on SCUMMVM.

I haven’t tried it but also there’s apparently a Plex port for Onion OS out there, so if you have a plex server or know someone who shares, you can download stuff to your Miyoo Mini Flip and watch it.