I built Armbian fresh using sid this time as I have had zero problems with "Bookworm testing". The new SD build still shows a severely performance-affected eMMC so I am pretending it doesn't exist anymore. When a new fast flash drive arrives, I will use nand-sata-install with that. I did note that with a bootable SD and a bootable eMMC, while nand-sata-install did copy stuff over, it would boot from the SD but mount the eMMC /boot directory. Kernel upgrades would upgrade the eMMC mounted root but a reboot used the SD card version. I will look to see if a manual change to the mounted root fixes that.
For the record, apart from the 1st 6 months in 2019 trying to use the vendor-supplied Linux (it was seriously flawed), I have been running Armbian successfully ever since. Any errors being of my own making.
Thank you to the developers.