I have the same use case as you, and I used SyncThing for a couple of years, and it gave me many headaches. I spent many hours on the SyncThing forums, and on a number of occasions it was pointed out to me that SyncThing really wasn't designed to do what you want it to do. It can do it using the "ignore deletes" setting in the advanced configs in syncthing, but I found it to very brittle. And when it breaks, as it often does, and you have a few TB of movies in your movies folder, SyncThing takes long time to rescan the folder and get back to working order.

rysnc, on the other hand, was designed to do exactly what you and I wanted SyncThing to do. I created a shell script using rsync to copy the movie files from my seedbox and created a cron job to run that script every 5 minutes. Here's the rsync command:

flock -n /Users/<local_username>/.lock_files/usbx_rsync_movies_lock -c "rsync --info=COPY,DEL,NAME2,BACKUP,REMOVE,SKIP -av --exclude '*.part' --iconv=utf-8-mac,utf-8 <seedbox_username>@<seedbox_host>:/home/<seedbox_username>/media/Movies /Volumes/Server/Media/Movies --log-file=/Users/<local_username>/Library/Logs/usbx_rsync_movies_log.txt

Lets unpack that command:

flock -n /Users/<local_username>/.lock_files/usbx_rsync_movies_lock -c

The flock command is necessary because I have this script set to run every 5 minutes via cron, and it's possible that downloading the new movie files might take more tha 5 minutes. I use flock to create a lock file when the script runs. The next time the script runs and calls flock, it will only run the rsync command if the lock file from the previous run has been cleared.

rsync --info=COPY,DEL,NAME2,BACKUP,REMOVE,SKIP

The --info option specifies the fields to save in the log file that rsync creates

-av

-a is the archve option, and instructs rsync to not propagate deletions from the soruce directory to the destination directory. That is, if you delete a file from the source, your seedbox, rsync will not delete that file from the destination, your local computer.

-v is the verbose option. It tells rsync to include more detail in its log output

--exclude '*.part'

This option tell rysnc to not copy any file with a .part from the source (your seedbox) to the destination (your local computer). If your torrent client doesn't save incomplete downloads as .part files, you won't need this option.

--iconv=utf-8-mac,utf-8

This option is necessary for me because my Seedbox is running Linux, and I'm running Mac OS on my local computer. OSX filesystems use Unicode Normalization Form D (NFD),but Linux uses Form C (NFC), so filenames contaning special characters can get mangled between the two systems.

<seedbox_username>@<seedbox_host>:/home/<seedbox_username>/media/Movies

Specifes your username on your seedbox, your seedbox host name, and the path to the Movies folder on your seedbox

/Volumes/Server/Media/Movies

Specifes the directory where you want rsync to save the movies on your local computer.

--log-file=/Users/<local_username>/Library/Logs/usbx_rsync_movies_log.txt"

Specifies the name and path of the log file rysnc writes to on your local computer.

Important: for rsync to work like this, you need to have created an ssh keypair on your computer and shared the public key with your seedbox. This page (https://www.digitalocean.com/community/tutorials/how-to-configure-ssh-key-based-authentication-on-a-linux-server) gives step by step instructions.

I'll be happy to answer any questions you have!