Check out https://openwrt.org/docs/guide-user/firewall/fw3_configurations/fw3_ipv6_examples#dynamic_prefix_forwarding as it has a trick where you can use negative numbers for the prefix.

Check out Dynamic Prefix Forwarding as it has a trick where you can use negative numbers for the prefix.

Configure a static DHCPv6 lease and add a forwarding rule:

uci add firewall rule
uci set firewall.@rule[-1].name="Forward-IPv6"
uci set firewall.@rule[-1].src="wan"
uci set firewall.@rule[-1].dest="lan"
uci set firewall.@rule[-1].dest_ip="::123/-64"
uci set firewall.@rule[-1].family="ipv6"
uci set firewall.@rule[-1].proto="tcpudp"
uci set firewall.@rule[-1].target="ACCEPT"
uci commit firewall
/etc/init.d/firewall restart