Hi @Raven,
I think you already noticed it but I posted some information regarding the possibility to write a Signal client for the Librem 5.
I I might find some time to spend into it soon, although it would not go as initially planned. A first version could be written in Java, using as much code as possible provided by Open Whisper Systems and/or the Signal Foundation. A better version could come later in C or C++.
Purism has also already set-up a developer portal for the Librem 5. It is also up to us to enrich the platform with Free Libre Open Source Software 