So, I updated Tumbleweed, and the updates to KDE caused my Plasma/Wayland session to restart, breaking the updates part way through. I wasn't watching at the time so took some while to debug!

Spent some time learning how to use nm-cli, because new half-upgraded KDE wouldn't load the network widget. It looks like something else may have changed and mucked up in the half-update (and of course I rebooted like a wise-man/dummy/i-dont-know-but-at-least-it-didnt-make-it-work) but iterations of trying things in nmcli eventually worked!

Finally tried zypper dup again and saw the session restart, so finished the job from the virtual terminal! At last, I seem to have a working computer again, and I might just brave updating my main laptop. (I cancelled the update while it was still downloading packages, after seeing the breakage on the other laptop!)