Custom IPv6 DNS in Router Advertisements

[ugdp3]

I have an ASUS RT-AX59U router running the official firmware (Merlin is not available), and a Pi-hole on my network acting as DNS. I have configured the router's DNS settings (both LAN/WAN) to be the Pi-hole instead of my ISP's DNS servers. This works well on IPv4 (DHCP includes my Pi-hole's private IPv4 address), however for IPv6 the router always declares itself as DNS in the Router Advertisements instead of passing along my Pi-hole's (link-local) IPv6 address. The router is effectively proxying all IPv6 DNS requests, and since most devices on my network prefer IPv6, this results in nearly all Pi-hole traffic appearing to come from the router itself. So basically, the "Advertise router's IP in addition to user-specified DNS" option only works for IPv4, but not IPv6. Does anyone know of any workaround, possibly involving custom scripts, in order to force the router to supply the correct IPv6 DNS server? Note that my ISP supplies dynamic IPv6 prefixes and as such I cannot have other devices on my network generate RAs.