i'll throw in one of my projects as a contender: ytt - YAML templating tool - https://get-ytt.io (check out live playground!).

it works with yaml structures (hence avoids text templating problems) and uses familiar python-like language, starlark, making quite easy to get started. it makes use of yaml comments to assign metadata/templating directives to yaml nodes, so it looks something like this:

  #@ load("@ytt:data", "data")

  #@ def labels():
  app: echo
  org: test
  #@ end

  kind: Pod
  apiVersion: v1
  metadata:
    name: echo-app
    labels: #@ labels()
  spec:
    containers:
    #@ for/end echo in data.values.echos:
    - name: #@ echo.name
      image: hashicorp/http-echo
      args:
      - #@ "-text=" + echo.text

it doesn't include type checking, however, it does have a system to "overlay" structures on top of each other via overlay feature -- https://github.com/k14s/ytt/blob/master/docs/lang-ref-ytt-ov.... merge/replace/remove operations expect to find one node by default so map key typos or wrong structural nesting problems are caught easily in common cases.