Could this be forked out by LineageOS?
What point there is to forking a platform where the upstream is as big as a mammoth as Google, changing millions of lines per release and actively breaking API compatibility?
Your fork will stay stuck in an API version that in a couple years will have almost no programs written for it. Who would use that?
Your only choice is to try to match the manpower from Google and try to keep your "fork" up-to-date. But that is also guaranteed to eventually fail as someday Google will change something that you literally _lack_ the manpower to revert.
Which is why I can't recommend an "open source" _Android_ phone. I don't feel promoting Android at all, even if open source, encourages my user freedom.