> IPv6s get constantly rotated by most ISPs

I've had the same prefix for five years now.

And yeah, sure, my device cycles through ephemeral IPv6 addresses often, but always within the same prefix.

Group IPs somewhere between /64s and /56 and you'll essentially get a household identity, at least for a few days to a few years.

Yep. The addresses in an IPv6 /56 or so are about the same as an IPv4 address as far as identifying a single location.