Something else to chip in here in terms of interesting ideas, specifically regarding “protected/limited (leashed?) boost”, there’s some angles to this that the Glitch fork of Mastodon seems near in terms of local only posts and hiding boosts from specific people.

On their own these features don’t amount to what OP’s describing, but I think they may offer some groundwork for ways to work out the “limited boost” idea. Also, the more I think on it, the more I kind of like Leashed/Moored as a description over protected, as I think it more clearly implies controlled reach of posts, but maybe that’s just me.