A company operates in the us-east-1 and us-west-1 Regions, with plans to expand to additional Regions in the future. Each of its two business units (BUs) has a VPC in each Region, and transit gateways in both Regions are peered, with the BU VPCs attached. The company intends to create more BUs and needs to isolate some BUs from others while migrating to an architecture that supports additional Regions and BUs. Which solution will meet these requirements with the highest operational efficiency?