I mean... Linux, specifically QubesOS and TailsOS, are the most private operating system on any Computers ever as of now.
But
If the Grapheneos team really want to have an entire ecosystem of OS alternatives that replaces any Google device's OS for more privacy, they can try an attempt on Google's Chromebooks as the closest thing to do. But another problem with this is that they're too weak and old, and no one mostly uses Chromebooks but for schools.
But an idea of Grapheneos and a laptop combining could have some pros with it. Add all the features Graphene has and use a heavy bulky/annoyingly normal looking laptop; this would in theory makes attackers and the police fooled my this method. Reason being that most of them would likely focus more on your phone than the laptop itself, since most people use their phones as the most active way of keeping personal data as means of covience. Funnily if they were try searching on the laptop, it would be an hasle for them and no connection between of a Google pixel phone known reputation and a laptop.
Maybe a Thinkpad, Framework, or a powerful enough pixel chromebook is reccomended for this... That is, if Linux isn't an option already.
Mighty fine idea though.