Viewpoint0232 I don't know how they did it, but when I gave my turned-off and bootloader-locked GrapheneOS phone to the repair shop for a new screen, they returned it with a factory reset and the original Google OS on it.
They pressed Power+Volume Down to power on the device into fastboot mode. Then selected Recovery. Then Power+Volume Up to show list of actions, then Factory Reset. After that they booted normally, and went through the GrapheneOS first time wizard to set their own PIN. Then they went to Settings, About, and clicked the version number 7 times, entered PIN, then went to Settings, System, Developer Options, and selected to do OEM unlocking. Then they rebooted the device into fastboot mode again, and ran "adb flashing unlock". Pretty much.
GrapheneOS does not hinder anyone from factory resetting your device, GrapheneOS will just guarantee that all user data from the phone is wiped before any access to fastboot unlocking is possible. Same on Stock OS. However, stock OS has Factory Reset Protection (FRP), which GrapheneOS does not. And this is the issue OP has. The factory reset protection means the first time wizard is forcing you to enter your old PIN from before the factory reset to prove you are you, making the device unusuable for others.