XMPP only does it with certain client extensions. And Matrix only does it when the rooms are set up this way. SimpleX does what you want, but is kind of unintuitive for the average user.
I say go with Signal, it does what you want and is idiot-proof.
Signal is annoying to use if you don't have a smartphone you can trust, since they do not allow registration from desktop. So either an Android VM or Signal-cli. But maybe it was just a one-off bug that the desktop client didn't bind to signal-cli for me. Still, the fact that you need an unofficial command-line application just to register makes it not exactly user-friendly.
I imagine that most people's families will find Singal easier than using a CLI program anyway. It's rare to find an entire family without typical cellphones.
Yea, but a typical cellphone is not as easy to make private as a typical laptop or desktop. Lineage has some tradeoffs and not accessible on all devices, and Graphene needs even more specific, quite expensive hardware!
Good luck getting grandma to install and use Graphene or Lineage on their 3 year old underpowered Samsung device that already works the way they know.
I think you're making problems where there aren't any. A device PIN or fingerprint along with Signal is probably way more secure than any family chat needs to be.
I meant telemetry to Google and/or manufacturer. With grandma, I can at least install Linux on her laptop and say to message me there (that's pretty much what I did with mom).