It doesn't really have to be any such fancy configuration dialect, even though I think they are much better than YAML with Jinja abominations you can find in Ansible or K8s Helm.

All that is needed are functions that can either directly contact the remote or build data model with which the tool will later do its magic.

See e.g. how you define infrastructure with Pulumi. It's just an example. I'm not advocating it. In fact, I have zero experience with it. But the point is, that it looks like imperative programming, but you're actually just building some model. You can employ language that's more functional. It doesn't matter.

Everytime someones mentions idempotence as a selling point of YAML abominations, they're mistaken. :-)

If he wants json, I will build it