I think avoiding a parser by using JSON is kind of neat, but it really isn't that difficult to write a parser for a small expression language and you get the benefit that you can expose a much nicer syntax to end users with the same parser - rather than having an standard AST and need a parser anyway to have decent UX.

I work on and off on a very simple syntax language for things like templates, but would be well suited towards business rules and such too. It was the first parser I wrote outside of school and pretty easy to understand, small and fast too: https://www.npmjs.com/package/jexpr