It appears you're attempting a parallel export using the --parallel flag to export SQL database to GCS bucket. However, parallel export is only supported or applicable for MySQL and PostgreSQL (see this documentation). This could be the reason you're not achieving the results you expect. Also, as stated here, exporting from Cloud SQL to a SQL dump file is not supported for SQL server.

For the equivalent SQL Server functionality, you can try to consider using the Striped BAK import/ export command or BAK files. For best practices, see Best Practices for Importing and Exporting Data for SQL server.