Port "forwarding" where packets destined for the router's IP are instead rewritten and forwarded to a private IP on the LAN side is not necessary under ipv6, what is needed is simply to open up the firewall to allow forwarding traffic to the public IP of the server as there are plenty of public addresses to go around for everyone (times several thousand trillion)
Suppose 2002::1 is your router, and 2002::2 is the server on your LAN you want to make available to the world, then create a new forwarding rule in LuCI, from WAN to LAN, click add and edit, select ipv6 family, allow any address in WAN to access 2002::2 on the destination port you choose with action "accept"
voila.
Now it's likely that you have issues with your prefix changing, hopefully not too often, but maybe if your power goes out for a day when your internet comes back up you have a new ipv6 prefix, so you'll want to check and edit this rule after such events. Some people have ISPs that haven't a clue or are actually malicious and change the prefix on a regular basis, if you have that your connection isn't suitable for hosting your own server, or you should use a tunnel.