>With Cue, I personally think it is uglier than TOML and Dhall and only slightly less reader-ambiguous than YAML

I think you're missing the main point of these languages.

YAML, TOML, JSON are all basically the same thing modulo syntactic differences, which are largely a matter of taste.

Dhall, Cue, Jsonnet, BCL, etc. are in a different league - they allow you to express some computations with data in your config. It helps you to eliminate boilerplate and duplication in your config, avoid copy-paste mistakes, make configs more concise, express some complex abstractions specific to your domain. They're miniature programming languages for your configuration data.

If all you have is a few dozen key/value pairs to specify, it doesn't really matter which you choose, just a matter of taste. But as your config files suddenly span thousands LoC, duplicate same things over and over, need to configure dozens of different things (e.g. with jsonnet out of a single jsonnet source you can generate multiple configs for basically anything json/yaml/ini driven), get edited by multiple teams and you start looking for better ways to structure this mess, that's when these more advanced config languages come in to rescue your sanity.