Thanks, so a feature that clearly shoots above any goal: 512 MiB for logging … and if /tmp is used for transcoding, there should be a way to move that to disk. I remember this was topic already, not sure whether in Jellyfin or Kodi, and a path on disk was used by default for this exact reason. Nothing else would/should be able to take even close to 2 GiB.
EDIT: And btw, no need to go above 2048 MiB for /tmp and 512 MiB for /var/log. Jellyfin does not actually use or reserve that size, it just checks whether in theory that would be available. For /var/log with hourly log clearing this will not even remotely be ever the case. For /tmp it depends on the video source and resolution, and whether this trickplay feature is used, which generates regular interval thumbnail images for videos. But other applications use /tmp only rarely for low size stuff which is usually removed after the task has been done. dietpi-software uses it for downloads up to 200 MiB or so, maybe 500 MiB for archive + extracted data temporarily, but then the Jellyfin service is stopped.
EDIT2: The logging dir can be changed via e.g. JELLYFIN_LOG_DIR=/mnt/dietpi_userdata/jellyfin/logs in /etc/default/jellyfin. But for the logs then some additional rotation would need to be done. A solution for /tmp could be a bind mount or symlinks /tmp/jellyfin => /mnt/dietpi_userdata/jellyfin/tmp or so. Maybe it also respects the TMPDIR environment variable. E.g. with TMPDIR=/var/tmp, maybe it uses /var/tmp/jellyfin, which is on disk.
For transcoding, it can be actually changed already: TrueNAS SCALE | Jellyfin
But not sure whether this affects the 2 GiB requirement 
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EDIT3: Ah that is for the TrueNAS SCALE Jellyfin app only.