Do I understand correctly from this thread that there's still no good one reliable way to send files across networks where both participants are NAT'd ?
What about using Onionshare to solve NAT'ing or at least Topr for handshaking?
Reliability is fine, performance is, depending on your demands, not. F the coordination server is not up to task of matching throughput of the two servers, and a lot of bytes need to be transferred, then it's noticable. But the tech at large works, depending on the implementation you choose to use. (I'm partial to Tailscale, although that's a different service, it lets me transfer files between computers (with additional SW) without manually mapping a port. They are using DERP for the coordination server.