I set up NAT66 recently with DHCPv6. The IPv4 and IPv6 addresses are practically the same, except IPv6 has a prefix and a double colon as the last separator.

This really should be how SOHO routers do IPv6 out of the box.

Most people don't want 1:1 addressing for their entire home or office.

Q on your setup

Are you using ULA prefix for the nat66/dhcp6, are you also allowing GUA address assignment via slaac? Im wondering how it works out with source-selection

Yes, so each device has two IPv6 and one IPv4 address.