Use Cloud SQL's built-in automated backups for your daily recovery window, and supplement them with on-demand (manual) full backups each month. Store those monthly backups in a customer-managed Cloud Storage bucket that you place in your required region - this satisfies the residency rule and keeps them until you explicitly delete the instance (on-demand backups aren't auto-purged). Automated backups handle the daily retention, and your manual monthly snapshots live as long as you need.