I know it's an anecdote, and I'm the founder of the Snikket project, but I rolled out an early version to 10+ family members during lockdown. The majority of them were remote and ages from <5 to >80. I emailed them the invitation link, and that was about it. They all managed to sign up with no issues.
Scanning a QR isn't actually necessary. That flow exists for when you open the invitation on e.g. a desktop or laptop, and you actually want to set up Snikket on your phone/tablet. The invitation page guides you through you what to do in that case, you don't need to know what a "QR code" is. But most people just open the invitation directly on their phone, and it has a magic link to install the app.
In some ways it's easier, because you don't need to go through the SMS verification and stuff that the phone-number-based platforms require. I know various APIs and hacks have made that flow easier over the years, but it's still an extra step for non-technical people to get lost in. It's also an obstacle for people who want to use a tablet, such as my children who are too young for phones.
In any case, I'd encourage people to be less defeatist about the adoption of more open alternatives (intended or not, that's how I read your comment). It is possible... my own family average no more or less technical than any other. They manage to use WhatsApp without help, and they manage to use Snikket without help. And of course I'm not the only person using it successfully.
My mother has no idea how to check her email and doesn't use it. She also doesn't know how to text. Or check voicemail. It's just another point of anecdata, but this is my bar for "non-technical."
I too have struggled to establish private communications with nontechnical folks.
I think the best bet would be to create a device with the sole functionality of establishing an encrypted voice line to you. Essentially, a plastic brick with an on button, speaker and microphone. No screen, no UI, just on/off. Like a walkie-talkie, but for infinite distances.
I don't know how it would have gone remotely, but no issue in-person. I opened an invite QR on my device, asked parent to open camera, and point it at code. They get the gist that the app name is Snikket, they get notifications of messages and calls, and the layout and icons are pretty similar to every other messaging app. I am going to attempt it with an in-law who only just stepped into the smartphone era this month, has no familiarity with a smartphone interface, and is the least technical person I know.
My dad finally retired his 2008 flip phone last year, but before that a QR code would have been no help to him. I visited them recently, and not a single restaurant (business at all) had QR codes in their town.
Scanning a QR isn't actually necessary. That flow exists for when you open the invitation on e.g. a desktop or laptop, and you actually want to set up Snikket on your phone/tablet. The invitation page guides you through you what to do in that case, you don't need to know what a "QR code" is. But most people just open the invitation directly on their phone, and it has a magic link to install the app.
In some ways it's easier, because you don't need to go through the SMS verification and stuff that the phone-number-based platforms require. I know various APIs and hacks have made that flow easier over the years, but it's still an extra step for non-technical people to get lost in. It's also an obstacle for people who want to use a tablet, such as my children who are too young for phones.
In any case, I'd encourage people to be less defeatist about the adoption of more open alternatives (intended or not, that's how I read your comment). It is possible... my own family average no more or less technical than any other. They manage to use WhatsApp without help, and they manage to use Snikket without help. And of course I'm not the only person using it successfully.