I’ve been using Fladder on my steam deck, it allows downloading for offline play. Works pretty well but the interface is too mobile centric IMO.
There was Jellyflix which had a similar feature but it wasn’t straightforward to get running on Linux. Haven’t checked it since!
I successfully did it about 2 years ago, following the instructions from this repo (last commit 2 years ago though…) https://github.com/MMMZZZZ/Jellyfin-Migrator