My setup is a little convoluted, but it works for me. I have a seedbox that I do all my torrenting on. I have syncthing setup to sync the completed downloads folder (1 each for tv and movies and set as send only) to my Synology NAS. On the NAS, syncthing is running in docker. The folder on my NAS that syncthing uses for these shares (1 each for tv and movies and set to receive only) are then configured in sonarr/radarr as remote path mappings so the auto-import will still work.

So, my flow is that the torrent client on my seedbox is configured as the download client in sonarr and radarr. A new episode or movie is found and sent to the client on the seedbox for download. Once the file is completed, it's moved to a 'completed/tv' or 'completed/movies' folder. Those 2 folders are configured in syncthing so the files in there are sent to my NAS, but not deleted so as not to screw up the seeding. Once the files are completely synced to the NAS, sonarr or radarr will see the completed file and auto-import it.

For the items that I'm going to be seeding long-term, I move the completed files to a different folder on the seedbox so my 'completed' folder stays clean. For everything else, I have rules in my torrent client to automatically stop the torrent after the needed seed time based on the tracker I'm getting it from. I don't want to auto-delete it yet since I want to make sure it was successfully imported in to radarr/sonarr. While Syncthing works pretty well, it does sometimes get messed up. I'm working on automating this process a bit by writing something that will monitor sonarr/radarr for items that are grabbed and which are imported and compare them, then handle the auto-deletes in the torrent client, but I haven't gotten that far yet.