Not what you asked for, but if you’re using Jellyfin only for music, try Navidrome. I used to have the same system as you: Raspberry Pi 4B, 4GB RAM, and Jellyfin was slow and clunky, eating up RAM. Navidrome is a service just for music (no video) and was much faster and responsive on the Pi 4B.
I know, but what about when I have several subtitle files? Different languages, or maybe several subtitle files I downloaded and want to check which one matches my video? mpv has zero flexibility.
With VLC I can just “Subtitle / Add subtitle track” or add the language code after the filename (video.en.srt, video.fr.srt, video.spa.srt), with mpv: just one file at a time: rename, launch, retry.
Until you want to add external subtitles
Fixed it for you