Okay, I figured out that "restart in safe mode" in the error page is actually a link - in very dark grey or black text so it's not at all immediately obvious. That allowed me to get into safe mode successfully, and update a plugin.
At that point, when I click on the "Disable safe mode and restart" link at the top of the app, the app kept restarting in safe mode, which was confusing - it turns out that the "flags.txt" file I created WAS in the right place but wasn't actually being recognised before? So after updating the plugin, it was recognised and forcing me into safe mode. I removed it and now I'm back up and running.
I'm going to leave the thread here for feedback, as I think there are a couple of things that could be improved:
- An out of date plugin managed to crash Joplin badly enough that the Help menu didn't load, and the "flags.txt" file wasn't being read, so neither of the safe mode options suggested on the website actually worked. That feels pretty fragile to me, although I'm not sure what the solution should be.
- The "restart in safe mode" link on the crash page isn't obviously a link, at all. If it was blue or underlined, this would have been resolved in seconds rather than taking me most of an hour to fix.