Plex already does this automatically, it’s called Plex Relay. If no incoming ports can be opened, traffic is tunnelled through Plex servers (max 1 Mbit for free, 2 Mbit for Plex Pass users).
I’m not entirely sure if you fully understand why ports need to be open, this is how the internet works, any externally reachable server has to either have an open incoming port to itself, or route its traffic with an outgoing connection to a 3rd party relay/VPN server (that in turn has an open port to the wider internet).
There’s nothing wrong with portmapping (or in the case of IPv6, firewall rules) either manually or UPNP/NAT-PMP/PCP, server applications have had it for decades.
There’s a feature request to extend the automatic port configuration for IPv6: Support for PCP (Port Control Protocol, RFC6887) - firewall hole punching
