From the application point of view, recently I'm converging on this: define data structures for your config. Ensure it can be deserialized from json and toml. (In Rust this is easy to do with Serde; in Python with Pydantic or dataclasses.) Users can start simple and write toml by hand. If you prefer KSON, sure, write KSON and render to json. If config is UI, I think the structure of the data, and names of fields and values, matter much more than the syntax. (E.g. `timeout = 300` is meaningless regardless of syntax; `timeout_ms = 300` or `timeout = "300 ms"` are self-documenting.)
When the configuration grows complex, and you feel the need to abstract and generate things, switch to a configuration language like Cue or RCL, and render to json. The application doesn't need to force a format onto the user!
When the configuration grows complex, and you feel the need to abstract and generate things, switch to a configuration language like Cue or RCL, and render to json. The application doesn't need to force a format onto the user!