Be that as it may, the Plex official guide for setting up "remote streaming" walks you through port forwarding. That implies that when they say remote streaming, they mean port forwarding by default. I then had to go digging to find mention of the Relay service which seems to be a fallback. (Apparently it isn't even supported by all clients)
Surely if they meant they'd start charging for Relays they'd mention that explicitly, and not use the term "remote streaming"?
In IPv6, a /64 is only supposed to be used for a single subnet. If you have a subnet smaller than /64, things will break. SLAAC needs a /64, which means Android phones for example can't use IPv6 on a subnet smaller than /64.
/64 might seem huge but that's just how IPv6 works. The entire 64-bit host ID is used for encoding MAC addresses into the IP address, or creating randomized privacy addresses. It needs to be huge so that it can do that statelessly.