With DeltaChat you don't even need an email address anymore, they provide it for you on the fly. They just ask your name if you (optionally) want to put it.

Can't be simpler than that tbh.

If you want a better looking ui, check ArcaneChat for Android. It's 100% compatible with DeltaChat protocol

Simplex is really easy to install and use, unfortunately it's still kinda buggy, specially with public relays, I personally don't mind buggy, I'm willing to make sacrifices for the same of freedom and privacy.

I just keep a second chat app as a failback so I can send them a message saying "ur simplex broke again, pls restart"

Xmpp has been stable for decades, tho I guess otr/omemo is hard for family to install, also doesn't support e2ee calls (or rather, it does, but it's complicated). But I haven't used xmpp in a long time.