I am sorry to say that forking AOSP is not an easy solution.
I cannot talk for Grafene as I don't know much of that organization, but I have been part of the Lineage Os organization for a little bit as a maintainer, and I can tell you that nobody is actually able to start working on such an effort as an AOSP fork.
I would gladly be happy if such a work would actually be maintained and supported long time, but I'm skeptical that anybody but a big organization has a power and a resource is to do so. After all even Linux is actually brought along by lots of organizations and also commercial organizations.
Yes, we need an open source trust platform, and I believe that is the only real way forward. I would vouch for a Linux mobile operating system, but indeed, air truly open, Android would be good as well.