Reddit見たら同じこと考えてる人いたわ、ここが参考になるかも知れぬ

https://github.com/mikeroyal/Asus-ROG-Ally-Guide/blob/main/Games%20and%20Software/Operating%20Systems.md

AllyにはSteamOSよりChimeraOSを推奨してる
Note: If you're planning to put Linux on your ROG Ally I would recommend using ChimeraOS