Looks a lot like Jsonnet. This is going into the direction I'm talking about.

Is there also some typing-checking/code-completion available for Kubernetes resources? I think this would be an essential part to improve developer experience and automatic checks.

Some linter to validate the k8s manifests must be a part of a solution. It's possible to check the output, but a perfect solution would also lint the RCL source code and highlight the sections that generate invalid manifests.