Is Matrix simple enough now that I can invite my grandpa/grandma and they will be able to join? How does ease up sign up and use compare to discord? Assume that I make the choice of which client to direct them to.
Just install Element on iPhone or Android and make a matrix account from there, then add each other by the selected username.
You will only have to help your grandpa when you start to add secondary devices that need to be authenticated the first time with a QR code (for decryption purposes).
I have been using Element to text my wife for around 6 months (Android to iOS). I took the opportunity when she was getting notification issues with a proprietary app to convince her to use it. She loves it now. Clean interface, no ads, and voice call works just fine for us.