Well rebuilt on a el cheapo 8gb card and all working.
Hopefully this one won’t corrupt although looking at the timing it was around an update it corrupt.
Hard to know what went wrong, especially when log files weren’t persisting…
I still suspect a bad SD card as a poor connection to the SD card in the slot can cause subtle file system corruption. You may not have even noticed this was going on until running updates, as updates generate a lot of disk writes and there is a lot of consistency checking done by apt/dpkg during the upgrade process - which means any subtle file system problems are more likely to be detected than they would elsewhere, such as a corrupt/missing thumbnail for a movie which could easily go unnoticed.