Solutions
-
NZLion's post in Plex disappeared and reinstall fails was marked as the answerwell... derp.
I found this thread which mentioned the advanced toggle in the settings, and I made the same mistake as that thread's OP.
Enabled Advanced View in the Plex settings page and I then had the ability to remove --runtime=nvidia from my configuration. Plex has now started successfully.
Leaving this here in-case it's useful to someone in future.