> but obviously Google still gets to see everything you do in their apps

Well, the actually scary part of google services is that they have this quasi-elevated access in your phone where it can do a lot of stuff ordinary android services just can't do. E.g. google maps' location sharing works this way (but don't quote me on that).

GrapheneOS managed to "put it back into the bottle", and it runs as a regular android service anyone could write, with the same rules applying. So you have much more control on what you allow it, and this will also limit what data apps relying on google services can leak about you.