If it's just Jellyfin, a SBC (like RPi 3B+ or better) running LibreELEC (just enough OS for Kodi) with the Jellycon add-on would do the job.

(There's also the Jellyfin for Kodi add-on which integrates your Jellyfin media into the native Kodi library, but my success with that has been limited.)

Configure LibreELEC to auto-connect to your phone hotspot, then you'll be able to control it using the Jellyfin or Kore app for selecting the media, and the Kore app for more advanced Kodi remote control. In my experience, at least, Kore is better for configuring subs and audiostreams, and for fwd/rev and fine-grained seeking.

Actually, I thought that Jellycon as the solution would make it impossible to select the media to play from the Kore app, but I just noticed that it's actually possible:

In Kore, select "Addons" in the sidebar, tap "Jellycon", then navigate to the "Content" tab, and tap "Jellyfin libraries". This means that you can control it all from Kore! - though, imo, the media overview is a bit prettier in the Jellyfin app, but I think the trade-off is worth it for a more sleek solution. You'll only need the Jellyfin app or webapp for forcing library scannings, editing metadata and such.