Why do you call these other languages “better”? They’re different, but I’m not sure why either of the one’s you mentioned would be better for this use case.

Modern config languages offer strong validation and advanced IDE support, which is essential for a great user experience.

https://pkl-lang.org/intellij/current/highlights.html

I was going to mention Cue, but I’ve only read about it, not used it, and couldn’t actually say whether it’s better.

I'm afraid that no current config language is an obvious fit for Mill. That's because Mill is fully reactive and doesn't distinguish between build configuration and execution by design.