Yes, Starlark is a relatively sane option, but the benefit of a language like that isn't simply that it's sandboxed, but also that it's not Turing complete beca

Yes, Starlark is a relatively sane option, but the benefit of a language like that isn't simply that it's sandboxed, but also that it's not Turing complete because it only allows for primitive recursion and thus is guaranteed to terminate. If you want a sensible compromise, it's to use something like Cue or Dhall to generate JSON configuration that's actually consumed by the software. There's also Jsonnet, but I've never been a fan.