OK! Booting!!!
So, the answer for me was:
At the point that you have mtd0 mounted, to erase and rewrite the SPI, so I ran:
/usr/local/sbin/rockpi_erase_spi_flash.sh
and then
/usr/local/sbin/rockpi4c_write_spi_flash.sh
(took about 15 minutes to execute)
Shutdown, unplug the SD card, and boot! I watched the blue activity LED with gloom in my mind, but then it came on, and I am now booted in! One weird side-effect is that I now cannot boot from SD (unless I remove NVMe - as in the boot order seems to have changed) but that is not really a problem.
I think I am all set - I am only at 300MB/s speeds, moving up to NVMe Gen2 (intfc:dtoverlay=pcie-gen2) didn’t seem to have any effect, but I’ll take a look at the drive to see if it’s anything to do with that - for the moment 300MB/s is absolutely great.