Regarding your edit and GrapheneOS support - they will definitely support the pixel for as long as Google are providing official support, though they have then continued support in the form of security patches for much longer than that for older devices. No guarantees of exactly what will happen in the future but you’re probably best of using GrapheneOS for now and then in three years time seeing what the state of things are. Things change quickly in technology, maybe you won’t need to move anything, maybe you’ll want a completely new phone by then!