Looks okay but I was looking for something that wouldn't require learning yet another config language and would rather take k8s yamls that I already have (or require minimal modifications).

Check out https://www.signadot.com/. Full disclosure, I'm the founder, but it could help with what you’re looking for.

> Define Sandboxes using a simple YAML file, specifying customizations relative to the baseline environment. Maintain these YAML files in your git repository and standardize Sandboxes across your organization.

It looks like these yaml files are not k8s files that I already have?

Also, is it open source? (I couldn't find a link to source on mobile)

Yes, these are our (thin) YAML files via which you describe the Sandboxes in terms of deltas from the baseline env. The K8s yaml files remain as the source of truth for your standard deployments. The operator is not open source. Some components like the CLI and Resource plugins are. We do have a free tier, however.

Okay, thanks for the explanation!

Checkout Skaffold and kustomize as well