

SLAAC won’t work with a smaller subnet. Static addressing is not an option since your /64 is going to be dynamically assigned. That leaves DHCPv6, which won’t work with any android devices or chromebooks.
It would be best to just run IPv6 on one network if you can’t at least get a /60.





Modern operating systems don’t use the MAC address for SLAAC. They generate a completely random address. You can choose a stable address or a temporary one that will change frequently for more privacy. You can also use both, IPv6 allows multiple addresses on one device.