And to answer the OP more directly, what do you give up by using a GrapheneOS (besides being forced to buy a Google device), well, no much since you have the option of installing Google services in a second profile or compartmentalized in your main profile. This, of course, would have your device communicating with Google's servers, but the info it can collect would mostly restricted. MicroG is potentially more private than creating a Google account (even if an anonymous one), but some claim is less secure too so I leave it as equal.

In conclusion, if you can live without installing Google services or juts just need it so sporadically that you only need to install it in a secondary profile, and you tolerate a Google Pixel go with GrapheneOS option. If, however, you are not a fan at all of having a Pixel, or need to have Google services constantly running, I would consider instead iodéOS, /e/, etc.