Can your router be configured to not advertise or offer an IPv6 address as DNS resolver at all?
IPv6 addresses can be tricky to associate with hostnames.
Instead of trying to identify IPv6 addresses, you could also consider to only offer an IPv4 address exclusively for DNS resolution in your local network (see e.g. Hostname resolution and IPv6 - #2 by Bucking_Horn).
Provided your router supports it, that would prompt your dual-stack and IPv4-only clients to send DNS requests only via IPv4, and IPv4 addresses are more easily associated with names.