A short while after I fixed Debian it ate itself again. This time leaving itself in an unbootable state. Rather than restoring from a stretch backup, I am doing a fresh install. So much changed between stretch and buster that it is not surprising the upgrade wasn’t entirely smooth.
It may take several days to get Debian back into a fully usable state. In the meantime, other machines which provide a similar modern Debian based environment include Ubuntu, Mint, JuLinux, Zorin, and MxLinux.