Threema is a nice piece of software but barely unknown outside Austria, Germany and Switzerland. It is Swiss-made and an entirely privacy focused chat software. The software is not for free but costs a few Euros. That’s why it cannot be found in the /e/app store for now. But they offer app purchase directly through their corporate website, so no play stores are needed to get hold of it.
What I personally like about them: They do not only provide a well designed and well working piece of software that is E2E-encrypted, they even have a comprehensible business model (providing messenger services to corporate clients).
Only downside, the code is proprietary - at least until now. Finally they announced soon to go open source with their client (hopefully the backend will follow).
https://threema.ch/en/blog/posts/open-source-and-new-partner