"Enable the experimental WSL 2 based engine" not selectable

1909 is a lower number than 19018. Also, I think you have to be in the Insider Preview program at the moment to enable that option.

https://docs.docker.com/docker-for-windows/wsl-tech-preview/

I'm in the same boat as you; I'm dying to use it as well, but don't want to join the Insider program, so I'm just trying to be patient, and wait it out.