This is surely the closest to a solution we have, IMO. An alternative to the Android-iOS duopoly that involves futzing with cables and command lines is never going to go mainstream. By contrast, a vertically integrated FOSS(ish) hardware-software stack might eventually get to 5% market share, like desktop Linux. At which point companies and governments will be forced to take software freedom slightly more seriously. That's my optimistic take.

Motorola will be releasing phones compatible with GrapheneOS, though it's unclear if they will ship with GrapheneOS.

https://proxysto.re/en/ sells GrapheneOS phones, with worldwide shipping I believe. They do donate a portion of every sale to GrapheneOS. However, you're recommended to install GrapheneOS yourself and not buy it from someone else, so that you know it hasn't been tampered with.

Because that would require a massive network and additional legal liabilities.

Partnering with Motorola seems to be the best bet.

They could partner with Fairphone and each can bring their complementary competencies.