Valve has launched another small update for the Steam Deck with SteamOS 3.6.6 Preview now available for testing which may be essential if you're playing ELDEN RING.
Fixed a an issue with a rare session crash during early startup of ELDEN RING
Fixed a general issue affecting all units on 3.6, and OLED units on 3.5, causing a slow memory leak during gameplay
Fixed a DSP firmware crash with previous 3.6 versions that could result in internal sound disappearing until next reboot
Improved responsiveness of session restart in case of session crashes caused by certain GPU errors
Fixed an issue that could cause videos to stutter in titles such as BlazBlue Centralfiction
Some pretty good fixes. Unfortunately you'll need to be on the beta/preview channel to get them, but hopefully it won't be long before they come to stable.
FWIW Plasma 5.27 works very well on Wayland with AMD GPUs. The fact that desktop mode uses x11 is probably not related to them still using Plasma 5. I would guess once KDE announces an LTS version of Plasma 6 (possibly as early as 6.2?) they'll upgrade Steam OS to that.
If they upgraded to Wayland they would have to rework their streaming protocol to function with Wayland and probably several other things need to be reworked. I use plasma 6 with Wayland on my endeavor install, but I still can't use discord streaming or steam in home streaming without problems. The x11 to Wayland video bridge by the kde devs isn't even updated to work properly.