I found that the Zig-Zag in Fox Trot looked better and felt better in open, rather than closed, position. Whereas, in the Waltz, it looked better and felt better in closed position.
Not sure if that is of any help at all, as you are a more advanced dancer than I, but for me it did make it feel and look more along the lines of what you describe as smooth, curving steps.
I do it almost like a pivot (on step 2) in the Waltz, but in Fox Trot, I take true S QQ S steps.
I will be interested to hear what others offer as advice