I tested Snapbridge for GPS tagging, it appears that the transfer of location data needs to be activated before the session and has a time limit (of two hours, I read this somewhere in Nikon's pages but did not verify). I noticed that although the bluetooth connection remains in effect, location data transfer is not happening unless refeshed from time to time from the application. I guess they are trying to manage battery consumption. Still, it's disappointing as the built-in GPS tagging works nicely in the D6 with no timeouts or battery consumption issues.
Since I would like to have a solution that works across all cameras, I went on and purchased a Garmin GPS device and am uploading regional open data maps that contains among other things, contours for elevation and boundaries of private yards. This is important information as I don't want to walk on someone's yard by accident. It has a lousy screen and 1980s style user interface but what is wonderful is that it can be used with a smartphone app that shows the same map and location info but on a big bright screen.
Track logs can be downloaded to the mobile or a computer. I will test tagging nef files based on GPX track log files when I find the time, probably tomorrow. It can use AA NiMH batteries which I also use in my Speedlights.
What is nice is the maps that this can work with, and that I can use it to GPS tag the files from any camera. I will still pursue automatic tagging via Foolography once it arrives hopefully in May (back-ordered). I will probably not use Snapbridge in my D850 to do this as while it had worked problem-free in wifi mode for quite some time, now trying to activate bluetooth took five minutes of frustration until it suddenly worked. I had forgotten about the hard time Snapbridge had given me early on. Anyway, I will continue to use it for wifi transfers, hopefully without problems as it has been.
(Edit) I now realize my mistake, the Auto link options contains Synchronize location data option which was not selected. I wonder why that changed, as previously I've had this data automatically tagged, but perhaps it has defaulted to having this feature off when the app has been updated or removed and reinstalled. My mistake. I'll give it another go! :-) Now it works.
Overwriting original GPS data based on GPX track files from the Garmin in Nikon Studio NX worked fine, and was easy to apply. Now I am not sure how the data is stored, if in sidecar files, is it something easy to read from a third-party application. I'll look into a search solution next.