As much as I understood the OEM will not build a device only and exclusively for the Graphene Team.
They try to find a partner who will meet their requirements in one of his mainstream models hardware wise and who also allows to install alternative ROMs like GrapheneOS and locking the bootloader plus give them the opportunity to install security patches etc..
That would be the only solution to find a device independent from Google and around the same price as the pixels or maybe a little lower.
The partner must be someone who is willing to sell a device that has at least some years of regular updates, because a throwaway phone without years of updates for the stock os will not find enough buyers.
My guess is that the OEM will be a big player with a broad lineup of devices who is selling in the EU. The EU has changed its laws that devices sold from middle of this year must provide at least 5 years updates and hardware for repair (batteries, Screen etc) beginning from the day the device sale has ended. Like they sell the phone until May 2026, it will have updates and batteries etc until 2031.