Couple of contenders:

- Jsonnet (https://jsonnet.org/) - simpler syntax and less concepts to learn, just an extension of JSON. But no type checking. An open source offspring of Google's internal config language (GCL/BCL)

- Cue (https://github.com/cuelang/cue) - a more ambitious attempt to fix GCL/BCL by replacing inheritance as the fundamental compositional primitive with constraint unification.

Great thread comparing them against each other by the authors of both: https://github.com/cuelang/cue/issues/33

Cue seems kind of similar to Dhall on first sight, but I haven't used either enough for an informed opinion yet.