With microG, your apps still connect to the Google servers, so you're not "breaking free".

Moreover, some OSes (e.g. /e/OS) give certain Google apps higher privileges than other apps even with microG, install Android Auto and it's still game over. GrapheneOS does not have this issue because as you say, Google apps/services get sandboxed.

Obligatory link: https://eylenburg.github.io/android_comparison.htm