I haven’t used it myself, but I’ve heard of ErsatzTV. Their docs note configuring Jellyfin as client.

https://ersatztv.org/docs/intro

If you’re looking for a companion app, you might want to try KDE Connect. You can pair the mobile app to your Steam Deck to allow for keyboard/mouse remote input and a bunch of other cool features. There’s also a Decky plugin for KDE Connect in Gamescope.