Sigma are nice stones, think dictum or dieter Schmidt stock them in europe.
Now you have the 1k, the 6k+ stone won’t be doing much work and so can be a softer type. I’d see what you can find for a good price. As you’re stropping on autosol, I’d stay 6-8k personally for most things I don’t see much difference between 6k and strop or 12k and strop unless it’s something particularly troublesome, and then it can be easier to scrape anyway.
If you’ve been happy with 6k from King then no need to spend more really.
Also worth checking if the Sigma can be soaked, some stones use a magnesia binder and can start to go soft if kept wet