Why do you open a new topic? You already found the other that has a possible solution: Threema via Chatty
It’s currently also the only messenger I use, so I hope for the bridge in above thread. There will never be partnership with Purism. Threema is proprietary software, and they don’t intend to change it.
The most likely way to have a “real” Threema instead of a Chatty plugin is via an Android compatibility layer. We’ll see when we get one…