You don’t want to download to JellyFin/TV. Keep the downloads in the original location and make sure hardlinking is enabled, the file will appear in both locations so you can watch and seed at the same time.

https://trash-guides.info/Hardlinks/Hardlinks-and-Instant-Moves/

From running sonarr/radarr in truenas core with plex and deluge ( previously I ran transmission) you can either point transmission to download into your jellyfin media folder, make your transmission download folder a jellyfin media folder or use sonarr/radarr to import your media from your transmission download folder to your media folder. Just set sonarr/radarr not to delete until transmission reaches a specific ratio.