Then your ISP isn’t following the RFCs. You might as well ask what you would do if you ISP gives your router a 17.0.0.0/8 address via DHCP. These are completel

Then your ISP isn’t following the RFCs. You might as well ask what you would do if you ISP gives your router a 17.0.0.0/8 address via DHCP. These are completely invented problems in your head. SLAAC can absolutely advertise a single /64 internally. It can advertise any /64 you tell it to. DHCPv6 can absolutely respond with DNS servers (and nothing else) in parallel. Configuring your SLAAC daemon to tell clients to get DNS servers via DHCPv6 is a 15 minute exercise with google.