Not sure if I'd use GrapheneOS even if it was available on other devices though. Really not a fan of the hostile attitude towards rooting when it's needed for basic functionality like backups — actual, reliable backups for every app, unlike those provided by the built-in solution.
Compare that to other custom ROMs, where you typically depend on volunteers maintaining the various devices. Sometimes people decide to step down, and suddenly find your device unsupported. This happened to me with LineageOS/CyanogenMod.
My understanding is also that the OEM ROMs of Pixel devices are closer to AOSP than those of other vendors like Samsung. This simplifies the maintenance of the ROMs, and enables the project to develop meaningful features instead.
We can't safely use a device where they might patch out support for what we rely on. As an example, OnePlus patched out support for alternate OS verified boot due to serious security vulnerabilities with how they'd implemented it. Operating systems relying on it would no longer be able to update the firmware, leaving them insecure, but yet the verified boot never worked properly anyway so it ended up worse than them not trying in the first place.
Realistically, what we expect is that Pixels are the only devices we're not involved in making which we'll be able to support for the foreseeable future. To support other devices, we need a partnership with a competent OEM like Samsung. We can raise money to pay the usual licensing fees for platforms and then work with them where we have paid support so they aren't going to break things for us, drop update support unexpectedly, etc.
> To support other devices, we need a partnership with a competent OEM like Samsung.
How realistic would it be to have an official Graphene phone? Could you partner with an open-source friendly company like Purism or Framework to design and manufacture the hardware to your specifications? That would be the ultimate mobile device for tech and privacy nerds, for which I'm sure many would be willing to pay a premium over mass manufactured devices.
As much as I trust your vetting of Google devices, there's a strong incongruity between the mission of a trillion-dollar adtech company and yours that I just can't reconcile.