[soundwave106]

The bulk of the user data (your movies, paths, what you watched) seems to be stored in a com.plexapp.plugins.library.db SQLite file (that you find in Plug-in Support\Databases on the Plex Media Server data storage folder, on Windows it is %LocalAppData%\Plex Media Server and someone linked to a support question with the rest).

So I imagine you can get a pretty solid backup just by using SQLite tools on that one file (though backing up the entire Plex Media Server folder might also be nice to do every now and then to save other things such as plug-in data).

The DB, although obviously undocumented, is also an interesting place to poke around for looking at the data for your own purposes. For instance, it looks like the statistics_media table stores your watched show history.