I'm giving up on XMPP, Prosody and ChatSecure because I couldn't get it to work. The sad details are at the end of this blog post, if you're interested: https://alexschroeder.ch/wiki/2018-06-07_No_More_WhatsApp
There's also the part where I discover that I can't use the iPad or my Laptop to register a second Wire account without a phone number because this Purism laptop comes with PureBrowser which is an older Firefox. So I think I'm going to suggest Signal to friends and family.