Although, proof that a Sybil attack occurred doesn’t allow everyone to sniff-out/filter-out the fake votes… that’s the trouble.

You’d need to identify that valid votes and share them somehow, perhaps each city having a few hubs that broadcast votes if the humans are actually present. Max number of votes per city hub, based on city population. This way an attacker would need to occupy most hubs across the country to make a large dent, and could no longer change votes even if they do.