Well, with v6 you're down from NAT-hole-punching to Firewall-hole-punching, which in principle should be as simple as arranging the IP:Port pairs of both ends via the setup channel, and then sending a "SYN" packet in both directions at once.

Then, trying to use e.g. TCP Prague (or, I guess, it's congestion control with UDP-native QUIC) as a scalable congestion controller, to take care of the throughout restrictions caused by high bandwidth delay product.