Your audience_string is still a form of @id since it is a vocabulary term you define.
{
"audience": {"id": "https://w3id.org/fep/xxxx/everyone"}
}
The thing is, 3/4 of your audience_strings already have other identifiers which mean the same thing.
me==$.actor.idfollowers==$.actor.followerseveryone==https://www.w3.org/ns/activitystreams#Public
The only one that doesn’t currently have a mapping to an existing term is instance, but this could be done with a Collection containing all local users, like a hypothetical https://social.example/users/.