Buy the device that is most affordable to you from GrapheneOS's FAQ "Which devices are recommended?". GrapheneOS's FAQ "How long can GrapheneOS support my device for?" mentions the following:...
GrapheneOS can only fully provide security updates to a device provided that the OEM is releasing them. When an OEM is no longer providing security updates, GrapheneOS aims to provide harm reduction releases for devices which only have a minimum of 3 years support.
Since the OEM minimum support length for GP6 is 5 years (source), my guess is GrapheneOS will not provide harm reduction releases for GP6. I recommend buy a used A series phone.
Buy the device that is most affordable to you from GrapheneOS's FAQ "Which devices are recommended?".
GrapheneOS's FAQ "How long can GrapheneOS support my device for?" mentions the following:
Since the OEM minimum support length for GP6 is 5 years (source), my guess is GrapheneOS will not provide harm reduction releases for GP6. I recommend buy a used A series phone.