Sabrent Rocket installed (Very easy) and Windows 11 installed (third try, get onto this in a minute) and it is working well. Need to do more tweaking but so far so good. SteamOS nicely tucked away on the SSD that came with the Deck so can easily switch back if I want to.
Now onto the fun.... Whilst Win 11 installed fine I simply could not get anything more than one monitor running with the latest APU drivers. Initially I thought this was my installer (custom to allow install on older devices), but retrying with both the normal 22H2 download from Microsoft, and my other installer used previously for the External & SD card installs, the problem persisted.
I even went as far as to boot up the external SSD install to see if I was going mad. Nope, 21H2 install of Windows 11 and the latest APU drivers.
Finally I decided to try the initial APU drivers and presto it all worked. I then re-installed the latest ones over the top (not a clean install) and it was still working.... Until it wasn't.
The issue I was experiencing was the two separate monitors (via Dock) were detected but not active (greyed out in the multiple monitor selection). Normal options of simply selected them and choosing "Extend to this display" just led to it reverting straight back. I could display on any one of the monitors (thus confirming the screen was detected) but not more than one. Attempting to troubleshoot this very annoying problem just lead to the usual useless "guides" or "solved answers" that don't help at all.
Back on the old APU driver for now but I did find another user who is experiencing the exact same thing:
www.reddit.com
One the plus side I haven't experienced any issues using the older driver for now and in performance in the games I want to play hasn't changed. Also the whole Windows experience is extremely snappy and only bogs down when the CPU comes under full load (note - don't try and do anything when downloading games on Steam @ 70+ MB/s (900Mb connection) whilst also simultaneously receiving Windows updates and Downloading on Xbox app (GamePass). CPU was pegged at 3.45Ghz 100% the whole time....