It seems there are a lot of misleading ‘facts’ on the internet (who knew? :P). People claiming they can get plex on the DLNA server in Big screen meta app were just saying it theoretically without actually trying it.
The only way to get plex to work on the oculus quest 2 is to pay for the skybox app and then connect plex through that, making sure you have the correct beta version to stop it crashing. (The skybox version you’d need is the beta 1.1.0 version of the skybox app apparently.)
Unless someone has got the big screen meta app working with plex, I think the above is the only way to do it!