A family plan would be beneficial for when the person whom is in charge of IT devices in the household can have NextDNS enabled at multiple sites/houses/premises etc.
If one user wants to use their Pro plan on multiple sites, there's nothing stopping them. Do53 IPv6, DoH, and DoT work everywhere. The only inconvenience is if deploying with Do53 IPv4 is the only choice, in which case link ip is needed, and right now a configuration can only be linked with one IP at a time. A simple workaround is to just deploy and link different configurations at each premise, it's not like the regular user would have the family spread across more than five houses.
Linking multiple IP to one config is a bit more involved, the linking query must also include the premise identifier so NextDNS can unlink the unused IP for that premise, without it subscribers with dynamic IP will accumulate tons of linked IPs that might interfere with other users.