I'd love to use Jellyfin instead of Plex, but the LG webOS client has literally never been updated. The first public version from July 2022 is still the latest version, and it's not really there yet.
Honestly it's been a year since I tried it now so I don't remember everything. One thing I really feel is crucial is turning down subtitle brightness in HDR. Plex allows for grey subtitles, which in HDR look white, except not at 700+ nits. They literally light up the room in dark scenes, and it's extremely jarring in dark scenes (with APL of 1 nit or less).
The webOS app is just a wrapper for jellyfin-web, so any changes to web will be reflected in the webOS app without it needing an update. As long as you keep your server up to date, of course, since jellyfin-web is ran from there.