Answering my own questions here – after some more digging around on the forums, I found this thread:
And that gave me the confidence and context to try the most obvious thing – I dug out a microSD card, an adapter, and downloaded the current i.m8xmq image, wrote it to the card, and booted it.
Lo and behold, it came up with the setup wizard. I configured it, then ran the reform-flash-rescue on it, which now allows me to successfully boot without an SD card, off the eMMC, where I once again ran through the setup wizard.
Next step is to deploy an image onto the NVME drive, but that’ll probably have to wait till tomorrow.
Anyway – it’s all working as I expected, and obvious in hindsight, the image is similar/the same to what’s already on the pocket, so I can set this up pretty much the same as I have that set up.