This setup satisfies the organization's requirements while minimizing costs:
1. Bandwidth and SLA:
- A total of 20 Gbps of protected bandwidth is required, which means you need redundant Cross- Cloud Interconnects.
- Each interconnect in a zone is provisioned for 10 Gbps, ensuring that both zone 1 and zone 2 connections in Frankfurt together provide the required bandwidth while meeting the 99.9% SLA with redundancy.
2. Cost-efficiency:
- Deploying the Cross-Cloud Interconnect connections in Frankfurt (the closest location to the CSPs) minimizes latency and avoids unnecessary costs associated with using additional colocation facilities like Munich.
- Using 10 Gbps connections in two zones for each CSP balances performance and cost.
3. Cloud Router configuration:
- Deploying the Cloud Router in europe-west3 (Frankfurt) ensures the routes for VLAN attachments are appropriately managed within the region close to the CSPs. The global dynamic routing mode allows connectivity between the us-east4 resources and the CSPs without requiring Cloud Router placement in the us-east4 region.

Private clusters run nodes that only have internal IP addresses, and do not allow public IPs over the internet to access the control plane endpoint. Additionally, private clusters do not allow Google Cloud IP addresses to access the control plane endpoint by default. Using authorized networks in private clusters makes your control plane reachable only by allowed CIDRs, by nodes and Pods within your cluster's VPC, and by Google's internal production jobs that manage your control plane.
https://cloud.google.com/kubernetes-engine/docs/how-to/authorized-networks

Cost and complexity of multiple tunnel vpn is very high, also, dedicated interconnect is not required as required max speed is 1.5gbps. Also, direct connectivity is bogus verb, all the solution provide direct connectivity to your vpc instance, once connected through router.
https://cloud.google.com/blog/products/networking/google-cloud-network-connectivity-options-explained

Cloud DNS private zones provide managed authoritative DNS for Google Cloud resources inside the VPC. An inbound server policy lets on-premises resolvers send queries into Cloud DNS for those private zones. A Cloud DNS forwarding zone enables Google Cloud workloads to resolve on-premises domains by forwarding those queries to on-premises DNS servers, delivering managed, scalable, bidirectional resolution without manual record synchronization.