You can run Kustomize in a Nix derivation with inputs from Nix and apply the output using Terranix and the kubectl provider, gives you a very nice reproducible way to apply Kubernetes resources with the Terraform state engine, I like how Terraform makes managing the lifecycle of CRUD with cascading changes and replacements which often is pretty optimal-ish at least.

And since it's Terraform you can create resources using any provider in the registry to create resources according to your Kubernetes objects too, it can technically replace things like external-dns and similar controllers that create stuff in other clouds, but in a more "static configuration" way.

Edit: This works nicely with Gitlab Terraform state hosting thingy as well.