Only allowing posts between mutual friends is instant messaging, not a social network. Discovery, engagement, and platform growth comes from people wanting to hear from and interact with followers who they don't necessarily follow themselves.
i think you are trying to solve a problem that in my opinion should just be skipped. i don't want to be part of a social network where some algo decides what i see. all i care about is what my friends do and maybe the friends of my friends. and that's it. that was the golden era of social networks, when precisely this was just the norm until they discovered that they can make more money by messing with the feed. no incentive to mess with the feed is what i'd expect from a non-commercial solution like the fediverse. or - at least allow for configuring my feed. if somebody wants to be exposed to all sorts of people - do it. i don't.
That may work for you, but it does not work for anyone running a platform and dealing with the needs of all users. That requires real moderation for both legal and practical purposes, as previously described.
