I have a watchface on my Fenix 6 that shows BG, etc., however I don't get a graph of that data in the Garmin Connect Dashboard. It may be recorded somewhere, but I've not looked at the export data for some time.
I can see BG when I record an activity using my Garmin bike headunit (I guess the same would be true of the watch as long as you enable the relevant datafield on the data pages for the activity in question).
That doesn't really help you though, sorry!