My unpopular opinion is that config languages with logic are bad because they're usually very hard to debug. The best config language I've used is Bazel's starlark, which is just python with some additional restrictions.
Is gcl great? No. Cue and dhal have better semantics. Do some people abuse it? Sure, but still much better than helm templating.
I agree, it's better than helm templating. But it's not better than python!
I wrote up why we selected CUE here, with links to explanations from Marcel who explains it better than I: https://holos.run/blog/why-cue-for-configuration/
These comments in particular reminded me of Marcel's video linked at the bottom of that article, where he talks about the history of CUE in the context of configuration languages at Google: https://www.youtube.com/watch?v=jSRXobu1jHk
Python with z3 and some AST magic can support constraint validation.
In fact, type checking can be seen as a form of that. I have a fork of typpete, a z3 based type checker from 3 years ago that should still work.