> The obvious solution to this, a fully managed DHCP6 setup, does not seem to be well supported by dnsmasq, but it wouldn't matter even if it was because so many devices don't support DHCP6, only slaac.

Relevant reading: https://issuetracker.google.com/issues/36949085

Android Public Tracker - Support for DHCPv6 (RFC 3315) - Status: wontfix