Pop!_OS. I have always loved System76 and have one of their laptops, as well as an HP Dev One that I use as a daily driver. The convenience and tiling system of the distro is the simplest I’ve used so far and works perfectly. I used to run Arch but I just don’t want to deal with it anymore, honestly.
I have not used Plex in years, however, just the fact that Jellyfin is FOSS is enough for me to want to use it more. I am not sure what features Plex has that Jellyfin doesn’t, but, it does everything it can do really well. Honestly, just throw it up next to Plex and see how you like it. Installation is a breeze, at least with Docker.