[tor-dev] Probability of Guessing a v3 Onion Address

Hi I was wondering what the mathematical probability of guessing an onion v3 address that is kept secret.

Or asked differently: what is the entropy of v3 addresses if an adversary decides to bruteforce the entire keyspace?

I am struggling to come up with a usecase for authenticated v3 services when keeping an address secret has the same effect and one can generate multiple addresses for the same server and share them with different entities.  The degraded usability of v3 auth services compared to v2 is the reason I'm asking.