I take it your seedbox is hosted externally and your Plex server is hosted internally on your home network?
You could set up a cron job on your Ubuntu server to run periodically and pull files from your completed downloads folder using scp. You’ll want to set up an ssh key for authentication to do this.
Edit: I changed my mind about recommending scp for this But yeah you should just do sftp from the terminal on your Ubuntu server using a cron job. Both of these should be fairly manageable with a bit of trial and error.
Stopping the torrent and removing it from your list will depend on what torrent client you’re using.