I’m playing Medabots: Kabuto for GBA. It’s a pretty fun although simple JRPG. It has a neat unit customization system, enjoyable story and great art/music.
Combat is unique in a way that can be a bit annoying (you only have a partial control on who to target) but it’s something one can get used to and work around to some extent.

I also installed The Sims 2 (Ultimate Collection, not the Legacy one) but I didn’t really spend much time playing it on the Deck (I usually default to my desktop playthrough). Based on a short test I did manage to do it seems to be working and controlling pretty well so that’s promising at least.