Klasmeister In order to access my Google Family calendar through Google Calendar, I would need to log on to my Google account, which is precisely what I want to avoid by using GOS. So the entire detour via Aurora store, etc seems superfluous if I end up logging on to my real Google account anyway.
Installing the Google Calendar app anonymously via Aurora and then logging in to the app with your real Google account does indeed seem like a contradiction in terms. But there might be other options, such as using Google Calendar via the web in a separate user profile. Whether that would meet your needs or not depends on more detail than we have so far.
Worldwide, something like 20% or 30% of smartphone users have iPhones (source). So it's plausible that 70% or 80% of people thinking of GrapheneOS would be switching from a Google-friendly Android distribution, not switching from iOS. This may be part of why many people think of GrapheneOS as a "de-Googling" proposition, even though for some people it is a "de-Apple-ing" proposition instead.
If you are currently using a few Google apps on iOS, and are contemplating using those same Google apps on GrapheneOS, it is indeed the case that such a switch wouldn't dramatically reduce the amount of data you would be sharing with Google. It would reduce the amount of data you are sharing with Apple, but that's a different matter.
If you want to try GrapheneOS for a while on a secondary device, the Pixel 6 series have a fairly long remaining support lifetime and aren't too expensive. Or you could try a 5a (11 months remaining).