Cloud SQL is the safe default — every other branch needs a name.
Not general enthusiasm for a more advanced-sounding product.
Relational, 1 region→ Cloud SQL
Needs more raw perf→ AlloyDB
Global + strong consistency→ Spanner
Document, app-facing→ Firestore
Do not choose Spanner or AlloyDB because they sound more impressive — both carry real cost and complexity. Reach for either only against a specific, measured requirement.