You are the security admin of your company.

Use a Shared VPC to enable communication between all projects, and use firewall rules to prevent data exfiltration.

Create access levels in Access Context Manager to prevent data exfiltration, and use a shared VPC for communication between projects.

Use an infrastructure-as-code software tool to set up a single service perimeter and to deploy a Cloud Function that monitors the "implementation" folder via Stackdriver and Cloud Pub/Sub. When the function notices that a new project is added to the folder, it executes Terraform to add the new project to the associated perimeter.

Use an infrastructure-as-code software tool to set up three different service perimeters for dev, staging, and prod and to deploy a Cloud Function that monitors the "implementation" folder via Stackdriver and Cloud Pub/Sub. When the function notices that a new project is added to the folder, it executes Terraform to add the new project to the respective perimeter.