ytt is even more templating-yaml-with-yaml, so it all ends up being a bargain bin Helm. There's no reason to do this over just serializing plain structures into YAML/JSON/...
I'm aware, but I just don't understand the point of it. How is it in any way better than a purpose-specific configuration language like Dhall/CUE/Jsonnet? What's the point of having it look like YAML at all?
Also you can actually write pure code libraries in starlark (basically a subset of Python)