While I agree to avoid using Sandboxed GMS I strongly disagree with do NOT use statement.

My personal case

My main profile doesn't have them, I get some proprietary apps through AuroraStore which grabs the APKs directly from Google Play Store without using my Google account and most of the time it works (Aurora Store is a bit buggy at time).

There is some apps that I can't use without Play Services or if they are not installed from Play Store itself (FUCK DRMs!). For them I have setup a user profile with the Sandboxed Play Services, it stopped when I leave the profile and let me use these apps that I NEED more than absolute privacy or anonymity.

My recommendations

I strongly recommend to anyone to try using GrapheneOS without them, especially if you're already into multiple FOSS apps. The Plexus app could help you identify which apps would cause issues for you without the Google shit. Then if needed for some apps with no alternative that suits you, setting up either a seperated user profile or, if it's too unconveniant for you, a Private Space with the Sansboxed Play Store and Services installed. And if these two are still not conveninant for you just install them on your main profile, that sucks but that's the sad reality of Android. As stated by the GrapheneOS Team it will still be way better than stock Google OS or any other manufacturer flavour of Android.

Not everybody have the same threat model and using GrapheneOS will improve your security, privacy and control over your device even with these proprietary background services from Google. Maybe for your threat model (or ideaology) you shouldn't use them but that doesn't mean everybody should do the same. Privacy shouldn't be all or nothing, it's about power and control over your personal informations, and GrapheneOS is a wonderful tool to take back some of that from your phone, having Sandboxed Play Store is okay even if not desired and you can choose how you want it thus having control and power over them.