how about regulating anyone getting public money or government permission to broadcast and telling them that instead of holding accounts on third-party services

how about regulating anyone getting public money or government permission to broadcast and telling them that instead of holding accounts on third-party services, they must also publish via an open and standardized protocol. You don't see public office holders conducting official business from gmail and yahoo mail accounts, do you? They use their own official internet domains to conduct business. The same can be done today with the use of ActivityPub hosted on official domains.