I think the Google Play issue with the app not being available for your device might be caused by Play Protect, which isn't available on GrapheneOS. I noticed in the thread regarding this issue that most, if not all, users with this issue use GrapheneOS.
A user in the thread for this issue enabled play protect and it fixed the issue, but, again, it won't work on GrapheneOS
This might be worth looking into.
"Apps using the Play Integrity API or obsolete SafetyNet Attestation API to check the authenticity/integrity of the OS can support GrapheneOS by using the standard Android hardware attestation API instead and permitting our official release signing keys."