Well, I have spent many hours following the guides and hints around the web for a path similar to yours but in the end I give up.
The Windows installation on the Sonnet was erratic 99% of times, only one time I have reached the end of the install process but on the restart the SSD on the card was not found again, at some point I decided that I would feel much better for a safety way, like using the good old SATA-150 ports that comes standard with the MacPro.
I have both 5.1 upgraded from 4.1 and 5.1 native MacPros, none of them was able to finish a clean install of Win10 using the Sonnet, and believe me, it was really a pity because that card has amazing performances since it squeeze out all the speed an SSD can do and none of the other PCIe SATA controller I had was able to do better.
My final configuration is now very similar to yours with a 1Tb Samsung 970 EVO (a good bargain from Amazon) on a Kryo M2 adapter: solid 1400 Mb/s on OSX, only a bit slower than Sonnet on boot.
The Win10 install was performed in legacy mode without an issue on a WD Raptor I had around mounted in one of the four trays of the MacPro while the Sonnet Tempo Pro Plus is now sleeping in its antistatic wrap, waiting for a new destination: I am reluctant to sell it because it has also eSATA ports and someday I could consider an external storage attached to them.