Folks, I have finally figured it out.
Have a baby.
Since having a baby a week ago, all of a sudden everyone is willing to install a decent messaging app in order to receive pics of the baby.
We explained that we weren't ready for images of our child to end up in the wrong hands via non-private apps. Another thing was telling them that the one single friend who had already got on board with this had already been recieving pics...
It's been a conversation starter for many and I think seeing privacy from the point of view of a newborn has helped our family and friends understand it a bit more easily. Plus they've had to put up with it if they want any photos, so they will see it working firsthand.
So, if you want to have a baby, know that it can be a wonderful opportunity to help loved ones communicate more privately.
It also increases the sum total of love, community and compassion in the world and in your own life but that's a conversation for another community :)
Edit: If anyone has good tips on how to share a little one's journey more privately with those that care about them, please post them in the discussion.
We're using Session with friends and family because I think it works most reliably with people who use apple devices out of the box. We use Jami for each other because it's p2p (distributed) and endorsed by the FSF. I set up Jami on my mum's phone too. You can use your own push notification provider with it or simply let it run in the background if you want to run your phone without google or apple servers but still want instant notifications for messages and voice calls. Jami is the app I would most like to see succeed. I believe you can also use it on internal networks, which is a pro in terms of independence future-proofing
Thanks, I didn't know about these two !