Kubernetes CSI drivers are surprisingly easy to write. You basically just have to implement a number of gRPC procedures that manipulate your system's storage as the Kubernetes control plane calls them. I wrote one that uses file-level syncing between hosts using Syncthing to "fake" network volumes.

https://kubernetes-csi.github.io/docs/developing.html

There are 4 gRPCs listed in the overview, that literally is all you need.