Good news - you should be using Signal anyway
XMPP was the correct answer 20 years ago, and (with an encryption layer) it's the correct answer now.
Matrix and activitypub are also valid answers.
I set it up for my elderly parents who just got their first smartphone a few months ago, and it's been going very smoothly.
What are the complications that others seem to be having?
Seems like it has everything a non-technical person would want.
Yeah, but grandparents (or whoever you are trying to introduce to Matrix) are not supposed to set up and run a Matrix server. If you meant that as an end-user you are having issues with the server you are on, perhaps you should use a different one. I have been using Element for quite a while now with matrix.org as my server, and I did not have any problems with the server I am on. The client is indeed buggy a bit[1]. Some bugs have been fixed since then though. If you run into bugs, report them please.
[1] There are many clients now for Android alone, for example, from F-Droid. Perhaps they are less buggier. I do not know.
Again, there may be bugs. They will get fixed. It happens all the time with any software. Element is maturing. It is really easy to just download and use without any problems.
Edit: I forgot Google Hangouts which is very buggy and where it is almost impossible to add new contacts.
XMPP, Matrix (Element), Wire, Briar, Ricochet, whatever.
Huh? Just do it. Install it, then help her if you have to, similarly to how you introduced WhatsApp (or whatever else) to her, for example. They may not like change, but if they are capable of using another piece of IM software, they are capable of using any of the mentioned ones.
I installed Linux for many people who have never seen Linux their entire life. On some occasions I explained how to do something, and that was it.
I think the point is that you don't need to help her for apps like WhatsApp, they manage to figure it out because it's simple enough.
Why?
I was under the impression that these days the NSA and other government agencies prefer perfect security since if they can crack the security then other governments could too. By forcing imperfect security the US would give up a large part of its advantage.