Fairphone 6 does not fulfill the hardware requirements (secure enclave, MTE, etc.), nor software requirements (monthly firmware updates, etc.). So there is nothing that GrapheneOS can do at this point. The ball is in Fairphone's court if they want this, but Fairphone seems more interested to cooperate with /e/OS, whose CEO proclaims security hardening is for pedophiles and spies (thereby fueling the narratives that support chat control and such nonsense).

One of the underlying reasons is probably that the Fairphone hardware and software is developed/maintained by a Chinese ODM (T2Mobile), so the question is if Fairphone even has the know-how to make such a phone. Heck, they have failed to fix bugs that have been plaguing people for months (e.g. regular dropping of all connections when using IPv6 on certain routers).

Motorola is going to make devices that fulfill the requirements and GrapheneOS is cooperating with them.