GrapheneOS is more private than Google Android and iOS. As a long-time GrapheneOS user who also uses Google Android as part of my privacy and security through isolation and compartmentalization threat model, I find it makes a great secondary device where more privacy is needed. The issue with sharing your IP with random people who run relays should be self-apparent. There are malicious relays on the Nostr network. As always, know your threat model and act accordingly.