I think it was a corrupted Facebook update that caused the problem*.
Put card back in phone, rebooted.
Nothing. Bugger.
Rebooted again (it has sporadically worked on startup, so worth a try)
Card found!
Uninstalled BBC weather app from the card (first on list, easiest to ditch quickly).
Card self unmounted. Bugger, try again.
Rebooted.
Uninstalled eBay app.
Card self unmounted. Bugger, try again.
Rebooted.
Uninstalled Facebook.
Card useage immediately dropped from 22gb down to 5.5gb.
Uninstalled Facebook Messenger.
Uninstalled Wikipedia app.
Card has been running for about 90mins without issue. Before it would crap out in 30 seconds or less.
Going to try it for a few days before I try reinstalling the apps.
Photos now uploaded to Google photos (thanks torsoinalake) and backed up onto laptop/NAS. 😀 😀 😀
Interesting experience but not one I would care to repeat.
* this time, anyway…