Complete bans (at the home instance level) would require synchronization between the content provider instance and the authenticator instance.

What are you referring to as a ban? Complete bans already require synchronization between different federated instances. Sometimes the home instance of a user is unable to entirely delete the content of a user because of it.

Mod actions are caused by users comments on content, so the two aspects are closely intertwined, you can’t dissociate the content from the users.

Not really. Mod actions are over a community, not user history. They are perfectly able to remove user comments within their community, and since they are the authoritative source that controls whom it is spread to that has greater influence. That never stops the same content by the same user from appearing elsewhere.

At the moment, admins synchronize in a group to deal with toxic users, usually leading to the ban of those users on their home instance.

They would still do the same, but the “usually leading to the ban of those users” perhaps does more to reveal what your actual problem is than anything else. You and me will have to disagree, because admins should not be authoritarian figures, but should only have control within their domain.

• If they want to administrate over a group of users, they can have control over which users are and aren’t allowed over that particular group. They can issue their own warnings to users.

• If they want to administrate over communities, they can have control over which communities are allowed and how users are allowed to interact with those. They can remove users from those communities entirely.

The small but loud minority of toxic users can just have their authentication instances defederated if those instances refuse to do anything with them. If it is an authentication instance doing the defederation, then it will affect all of their users. If it is a content provider instance, it will affect all of their communities. In the current system, it does both because both are coupled into the same instance, so it’s even compatible with it.

It stops bad faith actors from trying to pollute communities to slur entire instances, like lemm.ee or blahaj, because of their problems with their userbase, by simply stopping it from being an issue. Administrators don’t have to worry about policing communities or users if they don’t want to, they would be able to better choose whom they are catering to without bad faith backlash elsewhere.

Almost nothing of the current structure changes, except that dedicated instances have the functionality they don’t need disabled. Both can still block each other to their heart’s content, and if your problem is having more “splits” - that is literally what federated instances are, there can always be more … Maybe your problem is with the fediverse and its distributed nature? You are making it out to be as if there is only ever a big bad group of toxic users and that all administrators always completely agree on all bans to make your argument work. At that point, just create your own reddit clone.