To address the main thrust of your question yes GrapheneOS shares parity or is better in comparison to iOS.
However based on your reference to the Android based alternatives you shoild keep the following in mind...
GrapheneOS and CalyxOS are much different. GrapheneOS is a hardened OS with substantial privacy and security improvements:
https://grapheneos.org/features
CalyxOS is not a hardened OS. It substantially reduces security. It recently went 2 months not shipping standard security patches.
Compatibility with Android apps on GrapheneOS is also much different. GrapheneOS provides our sandboxed Google Play compatibility layer:
https://grapheneos.org/usage#sandboxed-google-play
Can run the vast majority of Play Store apps on GrapheneOS, but not CalyxOS with the problematic microG approach.
CalyxOS is closer to LineageOS and they both always use multiple Google services too and give them privileged access even if users don't use microG. It would be wrong to imply they don't use Google services. microG is of course an implementation of Google services. GrapheneOS doesn't use Google services by default.
To clarify further they always use Google services even without microG. They use Google for connectivity checks, network time, attestation key provisioning, SUPL, DNS fallback (LineageOS only), PSDS (Pixel 6 and 7), eSIM activation and more enabled by default.
https://blog.privacyguides.org/2022/04/21/grapheneos-or-calyxos/ is a 3rd party article explaining some of the substantial differences between GrapheneOS and CalyxOS. It's a common misconception that they're similar. CalyxOS is far more similar to LineageOS than GrapheneOS. There are many other alternate OSes available.
https://privsec.dev/posts/android/choosing-your-android-based-operating-system/ is another article about privacy and security differences between alternative Android-based operating systems. Talks about other alternate operating systems including DivestOS. Unlike most content, these are based on real experience and technical details.