Threema is not open source yet so I wouldn't use it for that reason. I did like their security whitepaper, though, and they do have some impressive clients (Mercedes-Benz, etc.).

Additionally they have regular audits [1]. Open source only without any actual thorough reviews doesn't cut it in my opinion.

[1]: https://threema.ch/en/blog/posts/audit-2020-en

Open source is a precondition. They can promise you the world, but if you can't build it yourself and get reproducible builds, who knows what's in there.

Yeah, agree. We've had enough fun recently with the likes of the CIA backed Crypto-AG.

Sure, but a precondition itself doesn't guarantee anything. You still need to pony up the money for actual aufits.

Yep, they're still working on that.

It seems that Threema charges for the app instead of requiring a phone number. Do they offer anonymous payment options like cryptocurrency or cash by mail?

I'm not sure how data protection works with Google Play and App Store payments, but I feel like this could be a potential privacy problem.

You can buy Threema from within their own shop: https://shop.threema.ch/. They support Bitcoin.