" but these seem to be relatively rare nowadays "

AT&T 5G uses Symmetric NAT. It's not rare if you have an iPhone or iPad with cellular. No way to do P2P without relaying traffic unless you want to "guess" the randomized port number, and, on that front, there are NAT-device-aware algorithms that can make that process faster.

We were promised IPv6 will make NAT's not necessary but I believe service providers use NATs not simply to conserve the IPv4 space but to actively discourage using the service to host your own servers.