GrapheneOS has a lot of security features that make it basically impossible to compromise your phone

This is very dangerous thinking. No software is perfect; there are always ways to get in.

Now, in practice most people aren't victims of targetted attacks and even devices with dozens of known local privilege escalation and remote code execution exploits won't ever be attacked but those who will cannot rely on "x is basically impossible to compromise". It takes layers and maintenance to actually be somewhat secure.

Sandboxed Google play service is actually more secure than MicroG.

it also has a Google Services compatibility that would allow you to use Google services, which would allow Google to harvest a lot of data from you

So does MicroG. Actually that's kind of the point of both. Google play services exist to provide, well, Google play services like push notification which uses the Google servers. That's the case with Sandboxed Google play and GrapheneOS.

OSes like CalyxOS or Lineage have microG

Yes. Pre-installed. That's kind of a problem if you don't want to use them in a specific profile. Also it's quite a security issue since it needs signature spoofing to work which is insecure by definition.

has the ability to use Mozilla network location to stop google from getting that data

That alone doesn't stop Google from getting your location data. Only not giving the your location data does this. On GrapheneOS you do this by... not enabling the location permission for any Google services including the play services.

Also by default GrapheneOS already uses GrapheneOS servers for location data and you can set your own proxy in the location settings

That's basically everything about it. Yes, battery life is an issue on GrapheneOS with sandboxed Google play service, but well, at least it just has the permissions of a normal app. Not a privileged system app that pretends to be another app to work.

GrapheneOS itself is great, but the GrapheneOS community is toxic. Something to consider. If you want deGoogled Android without the drama, consider CalyxOS.

As you said, proper google services instead of microG. It's been a bit since I used it, but it wasn't great back then.

Sandboxed Google Play provides better security, privacy and usability. Why that is, as well as your question about Calyx vs Graphene, is answered in this Privacy Guides Blogpost.