grphnswimmable376 So only do i need is to connect to an open-, public wifi network some km far from my home.... that is it?
If you are "not a superhero" and your goal is "general de-googling my life", it is not clear why any cloak-and-dagger action is required.
Performing the "check-in" step via public Wi-Fi stops Google from associating your IMEI with your home address. OK, but then the phone will be wiped and apps installed under GrapheneOS -- even Google apps -- won't have access to your IMEI.
It might be productive to indicate what you hope to gain by going through the extra effort of OEM unlocking anonymously. As Dumdum pointed out, it is not a requirement for successfully installing GrapheneOS.