gos_user591
You should have better privacy on GrapheneOS, already solely due to having better security.
On iOS, Apple can also be expected to have system-level access just like Google has (through Play Services) on Stock OS, so I would equate them in this regard as well.
IPC is mostly a non-issue for most contexts and setups. It requires mutual consent from both apps and even if two apps agree to communicate, it does not enable arbitrary access to the other apps private data. Its pretty much like having a conversation with another person, every bit of information has to be shared voluntarily and the conversation can be denied or aborted by any of the two involved parties at any time.
IPC is also not specific to Play Services. It is a default part of the app sandbox and can be performed by any app without any privileges or user involvement. It is actually very important and required for things to work like they do.
With all that in mind:
Apps that agree to talk to Play Services can be expected to always be those that already bundle Google libraries, making no tangible difference. But even this does not mean that any important or valuable data is accessible to Google. Google libraries in apps don't automatically can access "the whole app".
For example: Signal includes Google libraries in its default flavor, for Maps integration and FCM (push notifications). No message content or sensitive data is accessible to Google, even though Signal uses Google libraries which communicate with Play Services.
The other way around - apps that don't bundle Google libraries, like many FOSS apps for example, simply have no reason to talk to Play Services and are almost guaranteed to not do so.