jsonnet[1] and kapitan[2] are the tools I currently use. Their learning curve is not optimal (and I tried to contribute to smoothen it with a jsonnet course[3] and a 'get started wit kapitan' blog post[4]), but once used to it it's hard to do without, and their combination makes them even more useful (esp. if you deploy K8s). In Ruud's case, Jsonnet might have been worth looking at as Hashicorp tools can be configure…
Kapitan founder here! Thank you for the promotion
Happy to help if you need anything