You don't need to convince your wife to
switch, you can start by helping her setup the app on her phone and tell her to use this app when talking to you.
Lower the stakes and the expectations. Even if she is starting conversations with you on any other app, you can still start the conversations on Element. There will be bugs and quirks she will find, you just offer help and ask her for patience.
With time, they get used to the app and will be okay with starting the conversation with you there.
That's what I did with my wife, my parents, closer family and friends. Then you get one day like the recent Facebook outage, and they will come to you to ask if you can help their friends to sign up to it.
Slow and steady is the way I found to run this race. I already managed to get rid of WhatsApp and Messenger on my phone, it's only telegram for the odd group chat that is still resisting a full change.