Unless you want to involve a 3rd party service, like cloud storage, you must setup a VPN tunnel between your NAS devices.
Both of them support OpenVPN (and PPTP, but OpenVPN are superior to that) server and client mode. But for the server mode to work, your Internet connection must include a valid Internet IP, i.e. not being behind ISP NAT. So you must check ...
The stream module is a layer 4 proxying module, that is, it only forwards TCP packets between the connecting client and proxied server.
As TLS happens on a higher level, nginx takes no part to the SSL encryption. I am actually surprised that nginx gives no errors on the ssl_certificate directives that are in your configuration.
Your only option is to copy ...
If you set up drives as concatenated, you will still lose access to data if one drive fails.
This happens because file system metadata (allocation table etc.) is on one disk, and without metadata you cannot know where files are on the disk.