Hooo boy. So last night, after work, I decided to 'start again'. I used my recovery USB to do the deed (reimage) and then afterwards I wanted to check that my SSD had only one working partition (out of the 8 that it makes). I had brain freeze and wondered why the boot manager showed my SSD twice. Those of you who only use SteamOS won't know what I'm talking about, anyway - so I just wanted to use my recovery USB to boot to check the partitions again (you can't alter the active partition, so you need to boot into recovery mode). That's when I found that my recovery USB that I've been using regularly won't work, the SD will either boot back into SteamOS (even if I select to boot from USB) or it'll just hang. I tried various permutations, dock no dock, direct into USBC with adapter etc, tried several other USB drives (they take a while to 'make'), I even tried a 8G microSD which others have said 'always does the trick'. It didnt' (it even sort of wrecked the card, which can't be recognised by any device anymore). There are a few threads that suggest that it isn't just my problem, some have even RMA'd their device (which might not be an option since I've cracked it open to put in the SSD). Booting from the microSD (Batocera and Windows) or USB (with or without hub, windows) was not a problem. So it didn't seem to be 'hardware' problem per se - the system didn't like booting SteamOS recovery. In between all of this, the SD itself played fine - I reinstalled Left 4 Dead 2 and played a bit. Windows 11 booted up ok from microSD. I contemplated installing a beta SteamOS.
Anyway, in the end I used a new-ish 128G SDcard and lo and behold, it booted quickly into recovery mode. It was late, so I just shutdown and will try again tonight. Hopefully it'll work again tonight.
TLDR
My take is that it seems that Steam Deck recovery is very fussy about the USB drive, though I can't explain why the drive that worked only 10 minutes before to reimage, wouldn't boot again into recovery mode. Fingers crossed.