de0u I very much expect that the OEM's devices that are running the regular Google-certified OS already pass attestation-based integrity and that their upcoming devices will also. But I do not expect that the OEM will be able to get Google to certify GrapheneOS on the same hardware.
Then I don't quite see the point of switching the manufacturer. Google phones have been good enough for GOS so far apart from this annoying Play Integrity API issue. Just my two cents.