I did, except for this bit that you added in an edit:
> You don't have to reuse the same certificate for several requests. You can get a new one for every request, for every person who is asked to verify their age and pays you $2, and if they're actually anonymous, there's no way to know you did this. Is a rate limit part of the proposal? Can I only sign up to one adult service per week?
This is trivially easy to detect at the attestation service. If someone is trying to repeatedly (and programmatically) use the same personal ID to generate attestations for different request IDs in a short time frame, you can throttle them, flag them, revoke their cert, whatever.
Again, the service host and request id is part of the certification request, so you can easily separate a legitimate signup for multiple different websites from suspicious multi-signups to the same service for the same govt id.
So the government can tell I'm signing up for pornhub i.e. not anonymous. Also pornhub would need a government approval to operate or they'd just block their requests (and possibly arrest me for using an illegal service). I'd think we'd want service providers to also be anonymous without requiring government approval.