route ipv6 dns to a destination of my choice

Does this mean setting custom DNS server (so devices using DHCP picks up what DNS server you want them to use) or intercept DNS requests (MITM or use firewall rule to drop outbound 53 port requests)?

I think most up-to-date OpenWrt routers can do later (with normal, unencrypted DNS requests), see openwrt.org/docs/guide-user/…/intercept_dns.

The model you mentioned (Flint 2) is supported by OpenWrt.

To clarify new android phone bypasses pihole with ipv6 isp box cant configure anything. Would this work?