Mediate? By being the medium through which you communicate.
At any point they might insert an advert, a bot, change the UI or the share features, some AI slop, etc. you will have no recourse.
Just by using their platforms they’re able to update their models of you, your family, your friends. The timing of chats, the data they have on you through Insta or FB, all flesh out and refine their model of you. You are doing their work for them, helping them get richer, all whilst they oversee everything you do.
As for alternatives? I already listed several. You rejected most of them for whatever reasons you gave. Those were primarily your choices rather than firm barriers.
Here’s some more options: Discord, Matrix, blogs +RSS, your own mastodon instance, mailing lists, FaceTime, Zoom, WhereBy, MS Teams, irc, Slack, Mattermost, a custom chat server you wrote yourself.
... which means you're a normal human being. Who would go to these lengths just to keep in touch with friends, over some deluded sense of "avoiding mediation"?
