Nicely written article and a good read! However I had not heard of Threema before. It looks like a promising messaging app itself, anyone use it?
I am using it to communicate with 3 people (our common ground as I don’t have an iPhone and don’t use Whatsapp).
A few years ago it felt a bit ruff and awkward to use, but many updates later it is as fluent as any chat app.
The security feels ok. Of course it would be a lot better, if they would open source their code.