Happy Eyeballs and switching to/from mobile are definitely hard problems to deal with in a privacy-friendly way, but I don't think IPv6 privacy extensions should really matter here. Just treat a /64 the same way you'd treat a /32 in IPv6-land. It may represent multiple users, but they're all sharing the same internet connection which is as much as the global internet really wants to tell you. Whatever's on the user-controlled side of the address is of course untrustworthy anyways.
