I thought about it a lot and actually ended up staying in Shimano HG after all. My original plan was to move up from the NX cassette as the quality has not impressed but found other use for the money and decided to stick with NX anyway. :)
I came across this and it should fit a formula hub. I might give this one a try.
https://webshop.bestbike.de/us/specialized-freilauf-fhb-fhb-formula-xd-drive-free.html
I have the same starting point and similar plans. Therefore really interested to hear how this turned out? Was this the right part and did you have any issues in installing it?