Google is anti-privacy of course, but it does some things to protect users of Google Fi that no other carrier does - https://www.gstatic.com/fi/wormhole/whitepaper-a00cc4732620f382da5b7aac2bcb6905f970ba6b.pdf .
AFAIK, Google Fi requires Google Play Services, Google account, and Carrier Services app to work on GrapheneOS, but does it require users to be logged into their Google accounts at the time of using Google Fi MVNO cellular connections? Once logged into Google account via Google Play Services, can't users of GrapheneOS just log out of Google accounts, uninstall Google Play Services, and continue using Google Fi cellular services?
Or maybe that PDF white paper is misleading and Google Fi doesn't offer much over what other cellular connection providers offer to its users?