NoCode: fly.io with its 6pn (out-of-the-box private networking among clusters in the same org).

DIY: envoyproxy.io / HashiCorp Consul for app-space private networking over public interfaces.

LowCode: Mesh P2P VPN network among your clusters with FOSS/SaaS like WireTrustee / tailscale.io / Slack Nebula.