@MichaIng, this might be just the kind of specialist knowledge I need! I had learned from online searches that the kernel and dtb had to be kept in sync, but I didn’t know the u-boot package had to be aligned as well.
There was a typo in the first line of your advice, that I found and fixed as follows:
$ sudo apt list -a linux-u-boot-rock64-current
$ sudo apt install linux-u-boot-rock64-current=21.08.1
For the next two lines of your advice, I took no chances and ran as root:
$ cd /usr/lib/u-boot/
$ sudo su -
# . platform_install.sh
# write_uboot_platform "$DIR" "$(lsblk -npo PKNAME "$G_ROOTFS_DEV")"
The bottom line took no time at all to run, but there was no error message, so I assume it did. After rebooting, all the diagnostics are the same as before. Unplugging the flash drive and replugging it later causes some kernel activity (seen in dmesg), but the final state is the same as before.
That’s the end of my report on the u-boot intervention; I’ll try the quirks next and respond separately about those.