I’d say syncthing is not really a backup solution.
If for some reason something happens to a file on one side, it’ll also happen to the file on the other side, so you’ll loose your “backup”.
Plus, what ensures you your friend won’t be going around and snooping or making their own copies of your data.
Use a proper backup software to send your data offsite (restic, borg, duplicati, etc) which will send it encrypted (use a password manager to set a strong and unique password for each backup)

And follow the 3-2-1 rule MangoPenguin mentioned.
Remember, this rule is just for data you can’t find anywhere else, so just your photos, your own generated files, databases of the services you self-host, stuff like that. If you really want you could make a backup of hard to find media, but if you already have a torrent file, then don’t go doing backup of that media.

What do you mean jellyfin uses the *are suite?
I have Jellyfin with any media in different directories as long as I try to match the format the documents mention.
So, as long as I can get the media in any way I can just put it in any directory and it’ll be added to the library.

Is it similar with Odin? Or does it directly fetch the media from where you want to download it?