Windows runs fine on Deck. Performance is similar to SteamOS. SteamOS advantage is the shader pre-cache, which you can get on windows with DXVK. And again, connection isn’t always stable for xcloud, especially if you’re out, which I assume someone who has a portable device is doing.
The best way would be installing windows, as then you can play them offline and it isn’t dependent on how good your current connection is
In my experience Steam Deck on windows works fine. Just use Steam Deck Tools, it will handle the controller inputs for you. I also personally use Playnite to open the gamepass games instead of using Steam since it’s just easier that way.
add mario kart wii GUI