I've now SOLVED this issue, just in case it comes up in a search, and someone else has the same issue.

Or where I can find the WinRE.wim and restore the recovery enviroment (presumably including the MyAsus tool, and the ability to reset to the ASUS factory image without resorting to DISM)? Has anyone come across this before, and if so, any idea how to resolve? I've searched online for various different methods using the 'reagentc' command to try to restore the recovery image, but I can't find where the 'original' WinRE.wim file is stored in order to reset it. However, even after doing so, Reset this PC does not work as it can't find a recovery image.

This restores Windows 10 to how it came out of the box, and indeed I still get the OOBE, with the Asus licence agreement etc. I am also able to boot from USB and use the Command Prompt and restore the SWM to the C: partition using DISM. Now, I still have the ASUS Recovery partition, with the 20GB or so ASUS.swm file. I suspect that altering the partition table has caused this problem, and defaulted back to the standard Windows 10 recovery. However, in doing so I've lost the ability to Reset this PC, and also lost the entry for My Asus from the recovery environment. I recently performed a fresh install of Windows on my 2021 G14, booting from USB, I deleted the C: partition, re-created it and installed Windows, all fine.

Recover deleted partition linux