Back on the flip side, this is what I had to do to resolve the apt problem (and upgrade to the latest LTS Ubuntu, 22.04 Jammy Jellyfish):
Change entries in /etc/apt/sources.list to old-releases.ubuntu.com/ubuntu
(not adding /ubuntu at the end was the mistake I made earlier, that caused this change to fail)
Deal with armbian-zsh dependency bug
This broken dependency was blocking my upgrade (and probably the reason I got stuck here in the first place). Solution:
sudo apt purge armbian-zsh && sudo apt install armbian-zsh
prepare and do upgrade
sudo apt update
sudo apt upgrade -y
sudo do-release-upgrade
(note the last command is not a verb in apt, as is incorrectly stated in some places)
Blocking processes
(run in another terminal, after prompting by the update manager)
pkill xlock
pkill xscreensaver
Then press OK
Configuration files
During the upgrade, I got prompted by the upgrade manager about the following config file contents (altered by clockwork Pi installation most likely). Looking thru them to the best of my abilities, I decided to keep all of them as is (default option). Not saying this is correct, but my DevTerm hasn’t halted and caught fire yet 
‘/etc/issue’
‘/etc/issue.net’
‘/etc/systemd/journald.conf’
‘/etc/systemd/resolved.conf’
‘/etc/sudoers’
‘/etc/logrotate.conf’
‘/etc/initramfs-tools/initramfs.conf’
keep local /etc/ssh/sshd_config
‘/etc/apparmor.d/usr.sbin.cupsd’
‘/etc/update-motd.d/91-release-upgrade’
‘/etc/update-motd.d/95-hwe-eol’
‘/etc/pulse/default.pa’
You will later be asked to remove obsolete packages, which is what I did.
After restart
Re-enable third party sources:
/etc/apt/source-list.d/clockworkpi.list
/etc/apt/source-list.d/armbian.list (may need editing to point to new Ubuntu version)
and any others that you still need
Notes
Whole process took around 4 hours, including a lot of waiting and prompting for the config files as mentioned above.
In this case, the latest Ubuntu release also happened to be LTS (long term support) one, which means that you’ll get 5 years to upgrade, instead of 9 months. If the latest one is not an LTS release, there are ways to configure Ubuntu to only jump to the latest LTS, which you may or may not wish to do.
As I understand it, the OS image I started with, 0.1a, is still the latest available for download and presumably also the one that’s shipping with new DevTerms. So out of the box, to use APT again (and for other reasons including security), any new customer would have to go through the steps above.