artturnip:

Would it be possible to add an option to odhcpd to prevent recursive DNS servers from being announced in a Router Advertisement. I have tried setting option dns '' in /etc/config/dhcp but the router still advertises itself as a recursive DNS server.

RFC 8106 does not require DNS server addresses to be distributed via RAs and itself states that
However, for networks that need to distribute additional information, DHCPv6 is likely to be employed. In these networks, RA-based DNS configuration may not be needed. Thus, when using DHCPv6 on a network with an OpenWrt router, it should be possible to remove the DNS server address from the RA.