Buy European

Paid means you’re more likely a customer than the product.

I can’t verify the security claims either way, but this article is a counter point to the one one linked: https://www.bleepingcomputer.com/news/security/threema-claims-encryption-flaws-never-had-a-real-world-impact/

"While some of the findings presented in the paper may be interesting from a theoretical standpoint, none of them ever had any considerable real-world impact. Most assume extensive and unrealistic prerequisites that would have far greater consequences than the respective finding itself." - Threema.