If one is running Docker Enterprise with Kubernetes in an on-premises private cloud, is it possible to add clusters in a public cloud like Azure?
On GCP, Anthos is a candidate.
You may have a look on their architecture and see if it fits your needs.
Anthos is advertised in most of the GCP architecture courses and offers integration between GKE and both on-prem(the on-prem cluster must meet some prerequisites or you can use the version provided by Google) and AWS Kubernetes clusters.
Istio is a service mesh and if I understood correctly your requirements, the multiple clusters and multiple networks models could be used.