Funny timing, I've been playing with Jellyfin lately too and I ended up forking a little project to bridge Plex into Jellyfin: https://github.com/ndieschburg/xtream-to-strm-web
It syncs your Plex.tv libraries (even shared servers you don't own) into .strm and .nfo files, so Jellyfin just picks them up as a normal library and you can watch everything from any Jellyfin client. There's a built-in proxy that hides the Plex tokens from the STRM files, and for clients that don't follow HTTP redirects (like Findroid) there's an HLS proxy mode so it still plays fine.
Still a bit rough but it does the job if you want to keep a single Jellyfin frontend and still reach the stuff that only lives on Plex.