As it turns out, even with a reliable connection between D850 and iPhone, Snapbridge is useless for GPS data. At least if one does not want to have the same location recorded for
two hours 
From the D850 Menu Guide, page 179:
"Select
Yes to download location data from the smart device and embed them in pictures taken over the next two hours ...". Another proof that wonders never cease to exist

I wonder how much collective effort would be needed to convince Nikon to make the built-in BT connection to work with BT GPS mice (aka GPS trackers). Pending that, question is if a Foolography Unleashed '09 dongle (with the D800 modification) will fit.