I left Insta the day FB bought it; closed my FB, twitter, and Google accounts a couple of years later; WA was the hardest to leave, I'll grant. Since I left, I've used: phone; email; Signal; Telegram; letters; post cards; meeting up in person; sms; Mastodon; tried a couple of crypto chats. There are so many options it's not worth worrying about.
In the cases of special interest groups (think school/club/street/building groups), I just miss out, or ask for updates when I meet people. I am a bit out of the loop sometimes. No-one's died as a result of my leaving. When someone did actually die that I needed to know about, I got a phone call.
Honestly... just leave. Just leave. It's not worth your time worrying about these kind of "what ifs".
How about close friends who live on the other side of the world?
Telegram and Signal are, to me, about as trustworthy as WhatsApp. Well, actually, nobody really uses Signal, and Telegram is about the same as WhatsApp so who cares.
Waiting to meet my friends once every 1-2 years is not enough. I want to chat daily with them, because they are my close friends.
Daily telephone conversations with a group of them? Nope. Snail mail? It doesn't work for daily conversation.
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.
In the cases of special interest groups (think school/club/street/building groups), I just miss out, or ask for updates when I meet people. I am a bit out of the loop sometimes. No-one's died as a result of my leaving. When someone did actually die that I needed to know about, I got a phone call.
Honestly... just leave. Just leave. It's not worth your time worrying about these kind of "what ifs".