Lol, Snikket/xmpp has been suggested multiple times, and it is as good if not better regarding "wide cross platform" support. To get the same with Matrix you basically have to use a web-client or Electron, while XMPP has very efficient native clients.
just looked it up and couldn’t find a decent client for ios. There was Monal but it looks more like a draft rather than a finished application. Things don’t even have padding or margins. The snikket one I won’t even mention, you can’t expect people to use something that gives them visual discomfort.
I originally suggested Monal to my friend (who is quite into iOS and really appreciates a well designed application) and she found the same, but then she tried Siskin, and was happy enough to use it to this day.