Looking at pricing, it's a little out of my budget. But thanks for the suggestion!
If Cloud Spanner is too big, then you'll almost certainly be well served by Cloud SQL (fully managed MySQL and PostgreSQL): http://cloud.google.com/sql/
Firestore is literally built on top of spanner. So yeah, that's the answer. It's definitely pricey at first glance but it's a fixed cost based on nodes, compared to say datastore where you pay for storage+reads+writes. Different use cases.