Create a full mesh of VPC Network Peering connections between all five VPCs. Make sure not to import or export subnet routes with public IP addresses. Add Cloud network firewall policy rules to allow traffic.

Create a series of multiple network interface VMs with an interface in each VPPlace the VMs in an instance group. Create an internal passthrough Network Load Balancer in each VPC with the backend of the instance group. Configure custom static routes in each VPC with the next hop of the respective load balancer. Add Cloud network firewall policy rules to allow traffic.

Create a full mesh of VPC Network Peering connections between all five VPCs with an export exclude filter for 240.0.0.0/4 on every side. Add Cloud network firewall policy rules to allow traffic.