This was the discourse which failed an upgrade partway through due to lack of space.
Per @Yuun’s suggestion see that I can click on the initial Currently Upgrading... at /admin/upgrade. It certainly didn’t look clickable.
There I see the Reset Upgrade button, which I assume would work, but haven’t tried until this troubleshooting thread is complete.
Let me know which would be more useful to try first - the reset button or flushall.