The tagged-union / function-call point is where I keep ending up too. Once a config needs “one of these shapes” or “this thing with these arguments”, data formats grow conventions on top (type: keys, YAML !tags, $ref), and every consumer reimplements them.

I’ve been building Forma (https://forma-lang.com), a small typed Lisp aimed at that zone. Everything is (head args...), so a tagged value and a call are the same syntax, and you declare the vocabulary yourself:

  (job nightly-export
    :schedule "0 3 * * *"
    :retry (backoff :max 5))

job and backoff are forms you define with typed holes. The checker reports errors at the line you wrote, even after macro expansion. The output is plain JSON (or generated Effect TypeScript, but could target any language), so the app reading it doesn’t embed the language.