"In this case we suggest you to please use a device which is entirely working on Android/iOS operating system"
GrapheneOS is an Android operating system. In point of fact it is fully compliant with every facet of the Android Open Source Project.
What you actually suggest is that I install a version of Android that gives Google root access to the device as that is what is required to pass the Play Integrity API checks. Those checks do not actually ensure a secure device, merely that the OEM is willing to install various Google apps at the root level.
/ what would be my response. But honestly, nothing will change until the lawsuits start.