The Final Fantasy pixel remasters are all old school turn based RPGs that can be played one handed on the steam deck. Certainly not all 50 games but UFO 50 has a bunch of games made for a non-existent console that are all pretty simple and I would bet a lot of them can be played one handed. Both Crypt of the Necrodancer and Cadence of Hyrule are rhythm dungeon delver games that probably work one handed too, technically so would the new game Rift of the Necrodancer but that requires more skill than I have