Just because Google has been generous enough (or inattentive enough) to allow Pixel devices to run alternative OSes is not a reason to avoid GrapheneOS. Also, the Graphene project is in discussions with a manufacturer to produce a non-Pixel phone running GrapheneOS.
And there are other AOSP derivatives which aren't restricted to Google devices.
Why would you buy a Google device as a response to Google restricting user freedom?
I seem to recall that Graphene's leaders maintain that Pixels are more secure when running Graphene than any other smartphone would be (when running Graphene), which is not surprising given how good in general Google is at security compared to other large tech companies.