There is no issues with using playstore to be honest, its sandboxed on Graphene and is pretty much the only reason i run it myself.
Many other users including myself just uses the normal owner profile and google services like any other phone but with an increased attack resistance and privacy protection on the base OS.
The GrapheneOS team has stated previously that the purpose of Graphene isn't to de-google, but they want users to have the choice to do so if they wish. Installs from the playstore are safer and more convenient, so if you have no reason to go full blown "super-private mode", just go with normal playstore.