[rglullis]

Syncthing (the most recent versions, at least) also allows for password-encrypted sharing. So even if you are syncing across a relay, no data is in clear.

[j0057]

Relays can't see your data, because the connections between your synchthing instances are mTLS encrypted.

Using an additional shared secret on a folder allows you to sync a folder to an untrusted device, which then itself only sees encrypted files.