Beside your custom changes, does it work for you at all?
It should work out of the box = via Web interface of GL.iNet. However, IPv6 over PPPoE seems broken unintentionally …
That works here. However, as noted in that other thread, you have to change to NAT6 currently. Or go for LuCI → Interfaces → LAN → DHCP Server → IPv6 Settings → both, Router Advertisement- and DHCPv6-Service: server mode.
On the WAN side? The default is a /56. Why do you need a smaller one?
GL.iNet, can you comment on that? Another user requested that, too …
There are DMZ and port forwarding for IPv4. However, the equivalent for IPv6 is missing. Essentially, the current support for IPv6 is about Web surfing, right?
To avoid that, see this thread …