This is a great summary. Also, see the GrapheneOS teams list of hardware prequisites. They have said themselves before that this is not a very high bar, but that there are just no other manufacturers focusing on security. Apple claim they do, and get recommended as second best option in the forums sometimes, but given the walled garden approach, it must be next to impossible to develop against, even if they unexpectedly completely open up their ecosystem.

They would probably also have to take a lot of criticism for their implementation approaches and their brand integrity would suffer immensely. Maybe there would even be some new undisclosed vulnerabilities to fix, like the goto fail bug breaking SSL encryption ten years ago.

Compare it to the new open source Nvidia drivers for Linux, they have taken quite a while to develop since Nvidia announced the release, and I don't know if they have yet reached the performance levels of the proprietary ones. Doing this for a whole Phone, given they even fulfil the requirements hardwarewise, will probably take a decade. And in this decade, ten new iPhones will be released...