Setup an rsync systemd job & timer on your plex server. I think that's be the most efficient method. Can also use a cron job if you're more comfortable with that. Either way, the rsync would be a one liner unless you have to map different source and destination directiories.

I setup syncthing from seedbox to my NAS.

Add a cron entry on your Ubuntu server.

0 4 * * * rsync -r user@seedbox:folder/ /my/plexfolder

This would sync the folder on seedbox to the /my/plexfolder every night at 4am.

Ensure that you have set up password less authentication with ssh keys.

You can leverage scheduled tasks to execute a command that would sync your SFTP. I don't know what that is on PC, but there must be something like Cron for Windows. I'm on my phone right now, but feel free to dm me for help. Maybe we can trade plex invites.

resilio or syncthing.

That should be easy enough to do with a cron job. What OS is your seedbox running?

If you are regularly syncing things you should set up sonarr/radarr. If you use these to organize your library and perform your downloads (using prowlarr/jackett/rss as an indexer). The *arr apps would be hosted locally and you’d connect your seed box download client to them. More information can be found in the docs for each, but the *arr apps would perform a file copy/move after the download completes. Then Plex would just pick that stuff up automatically along the way

Have a look into rsync. Can't remember exactly how I set it all up now, but it runs every night and syncs from my seedbox to my NAS.

I agree with using rsync.

Open a screen then use something like this command:

rsync -arvzip --progress {$remote}:/path/to/remote/files /path/to/local/files

If you are using a different port for SSH, you’ll need to pass an option like -e 'ssh -p 12345'

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.

WebDAV + Rclone

I use bitsync to transfer files from my seedbox to my home network. The app itself is years old, closed source but just works.

Theres versions for arm, windows and Linux.

Use it via windows 10, Ubuntu 20lts currently but did also use the arm version on my wd mycloud and multiple arm based boards like raspberry pi 2, pi 4, orange pi pc and cubietruck.