Yes. If you install Graphene and no play services your phone is de-googled unless you use Google services through a web browser or install another app that contacts them. Google does not have magical hardware level telemetry.
Yep! Since the Nexus phones, big G has kept dev's in mind when designing their devices. This is why there's so much control available on Pixies and previously on old Nexus phones.
Have a look at some of the threads on !privacyguides@lemmy.one for comparisons, my read on it is that GrapheneOS is trying to be as secure as possible which brings a lot of privacy with it, and CalyxOS is a bit more permissive because they're just trying to degoogle. What this means is that only Pixels meet GrapheneOS's standards whereas CalyxOS will run on other devices, and CalyxOS uses MicroG to spoof Google Play Services while GrapheneOS comes without any but allows you to install a sandboxed version of GPS if you need it.
You can have a read though GrapheneOS's website to get the pretty in-depth details on what it does, why it does it, and how to do it yourself. I've been using it for a couple of years now and I'd recommend it, happy to answer any questions if you need!
Ironically, they're some of the last phones to even enjoy decent custom ROM support. Even newer OnePlus phones have no custom ROM development going on last I checked