FSF does not get in the way of security. FSF believes source code should be publicly available in order to even assume the software is secure or private. In a perfect world that would be nice. But in the real world, proprietary blobs are required to make the hardware functional. As long as OEMs are removed about open sourcing the firmwares, both GrapheneOS and GNU are right in their own way.
SimpleX is not a Signal fork. It is it’s own protocol, service and app. It just utilizes Signal protocol for encryption like every good e2e encrypted messenger out there.
SimpleX allows anonymous identity, federation between servers and still a good UX.