User-specific, time-dependent state currently in a single MySQL DB makes schema evolution difficult. Which storage option fits best?

Cloud Datastore/Firestore is ideal for user-specific, time-dependent state because it's a NoSQL document database, offering schema flexibility. This allows for easy evolution of data models without complex migrations, which is crucial when dealing with varying user states over time. Its scalability and ability to handle high read/write loads also make it suitable for dynamic user data. Cloud SQL is a relational database, and while robust, its rigid schema would make frequent evolution difficult and require downtime or complex schema migration strategies. Cloud Storage is object storage, not a database, and is unsuitable for structured, queryable application state. Cloud Spanner is a globally distributed relational database, but its strong consistency and relational model still present schema evolution challenges similar to Cloud SQL, and it's generally overkill for user-specific, time-dependent state unless global consistency across regions is a strict requirement.