but honestly I think this one is much better without. The darkness of the foreground I think makes the sky seem more powerful and honestly there is nothing of interest or anything that would add to the picture in the areas where you can't see any detail
but about the ND grad. is .6 ND grad enough or should I go for something "stronger?"
You selectively lightened part of the meadow to make the flowers show some more, didn't you? It looks as if a ray of sunshine hit that part of the meadow where there IS no ray coming out through the clouds.
And do I see right that the sky is quite noisy? At 500px in length it is hard to tell, any photo can easily look good if shown that small...