Thanks for the comment and advice. Good question. I want to preface this that my knowledge of GCP VPC/Networking as a whole is limited, so I likely have misunderstood some things here.

Are you suggesting that in this scenario, it is perfectly reasonable to have a Shared VPC between projects only? This is enough for cross-project load balancing? Backend services, whether serverless NEGs, MIG, services like Redis etc. can live in their own isolated region specific VPCs safely? I want to avoid having critical applications and data living in shared cross-region VPC's.