Some untrue assumptions people (including me) have learned from interacting with commercial sites and social media and taken to the fediverse:

The idea that if you post something that is copyrighted, you are the copyright violator. Loads of people post stuff on Twitter and Tumblr that they've copied from other sites and they just don't care, which is their prerogative. But on the fediverse each post is copied to and shown by many other servers run by other people. All these people are now also the copyright violator and can be contacted by lawyers from all kinds of companies.

(most platforms contain settings that the admin can set to make sure external posts are not shown to people who aren't logged in. I highly recommend making these settings)

(also note that Google doesn't know which posts belong to which server. Your own post will show up as belonging to some other server, and the server you run yourself will seem to host some very bad content. To prevent this, besides doing ample blocking, you should also make the same settings mentioned above)

The idea that the database administrator of the server you use, the one that can see everything, is somehow a trusted individual. In commercial social media and sites these people are screened by the city, the policy or even the security services depending on the kind of data involved. And these people are monitored by their companies. Here on the fediverse, anybody can be an admin. There's no control, so from the user there should be no trust.

(so don't post anything that you wouldn't want to become public, including when you are using those non-public posting options, including when saving a draft)

A minor thing: we assume that retweeting (sharing, boosting, repeating, whatever) is ok, because we do it all the time on Twitter. However, it is only ok there because the Twitter terms you accepted had a clause that allowed others to retweet (and embed) your posts. Most fediverse terms forget to include this, so in theory boosting etc is illegal. But I think we can all assume that the intent of every poster was to allow sharing, so this is only a minor misunderstanding.

(it is why I mention in my profile that my posts are cc-by, which essentially covers re-sharing using appropriate mechanisms)

I'm thinking currently that the problem with social media is the comments / replies (assuming the DM thing can be closed properly as is the case with most social media nowadays). It's there that random people (and bots, spam accounts, trolls) have access to your experience of your conversations. This is what you see inorganic accounts targeting.

Usually it's like this. You see someone making an interesting point. It indicates there are many replies. You click to see whether people have additional information / points to make, only to find loads of tangentially related posts that accounts likely insert below anything that has matching keywords. Or all these 'have you also seen this link' things that are not at all aimed at the OP but at their audience.

Not having visible comments at all (old Tumblr) or being able to close comments (Instagram) or limit them to specific sets of people (Twitter) may not be a complete solution. Somehow you should be able to start an open conversation and not allow others to hijack it.

I'm now thinking of something like this: someone posts an OP / Top Level Post and becomes the owner of that conversation. Anybody can reply, but that only goes to the original poster. That original poster has different inboxes for DMs, replies from followed people, replies from followers, replies from approved persons, replies from all, etc. They probably get a notification for 'replies from followed people', but only look at the others like 'replies from all' when they feel up to it. I'm thinking of different inboxes for each Top Level Post, which you can also close after a while.

When the conversation owner sees a reply, they can click whether they consider the reply relevant or useful for public viewing. Only then the reply becomes visible under the conversation for anybody opening it. Obviously, if a conversation gets old, heated or boring, OP will just not look at replies for that conversation again. If someone tends to receive harassment online, these are hidden in the 'replies from all' inboxes that you don't need to look at.

So each Top Level Post essentially is a forum with its poster as moderator. But a lazy moderator, with no peer pressure to judge all that comes in, it's your own space after all. And no forum rules to reject posts, only to maybe accept a post that you feel adds to the conversation.

I'm not seeing how this would be compatible with Activitypub, though...