Yesterday I switched my living room PC from an old, dated HoloISO install (from before it switched to immutable) to Valve's SteamOS 3. It was smoother than expected!
Just grab the latest Steam Deck recovery image (which is from 2023...), flash it to a USB drive, boot it, open an xterm and run the recovery script. It will completely wipe /dev/nvme0n1, but that's where HoloISO was for me anyway (with Windows 11 on /dev/nvme1n1).
Once imaged, it should boot if your hardware is compatible. I'm using a 12th gen i5 with a RX 6900 XT and 32GB of DDR4, YMMV. However a couple of services were failing and boot time was slow.
I had a third NVMe drive in a PCIe slot adapter that SteamOS was having trouble with (causing it to get stuck on lvm2-monitor.service) so I removed it and that was sorted. There was also a service related to logging Steam Deck hardware info, and one for managing Steam Deck firmware, both of which I simply disabled.
Lastly, I had to modify Grub. I wanted a menu restored to be able to boot back into Windows when needed, with a timeout to auto-boot one of the operating systems if nothing was pressed. That was just a matter of reading through the contents of /etc/defaults/grub* and /etc/grub.d/* and reverting some of the changes such as screen rotation, resolution settings, etc. I'm somewhat familiar with Grub so it went smoothly.
One more change I did was get a Bluetooth 5.3 dongle for my Xbox controller. I now have the controller paired with the Xbox wireless dongle in Windows (and have disabled Bluetooth in Windows) and use the Bluetooth dongle in SteamOS with the same controller. You can turn the controller on by pressing sync for two seconds for Bluetooth mode, or press the middle button for the regular wireless mode. I did this because installing the xone kernel module on an immutable distro seemed like a bad idea.
Also, brew is a good package manager for SteamOS, for the odd program without a Flatpak (eg. pass). #SteamOS #LinuxGaming