PiHole broadcasting itself as IPv6 gateway

[Bucking_Horn]

For that model, we have figured a configuration that would not advertise an IPv6 DNS server address.

@Bucking_Horn

Try setting DHCPv6 Server to Off, and correspondingly for your RA Service, setting the M and O flags to Off (indicating that "Managed address configuration" ( Stateful DHCPv6) and "Other configuration" (Stateless DHCPv6) isn't available in your network).
This would have your router instruct devices to exclusively use NDP/SLAAC/RA to join your IPv6 network and hopefully provide no IPv6 address for DNS.

You should also switch DNS Delegate Type to Manual and not set any DNS servers.