MicroG has two huge advantages: you can inspect the code it's running, and you're not tied to Google services. For example you can replace the location service with a privacy respecting one. With the sandboxed play you can't do that, you're stuck with the Google version. The only thing you still need Google for is the push notifications, because the backend of the apps only speaks to them.

It would be amazing if grapheneos would support microG as an option. But they are too much "not invented here" for that to happen.