I find that much of the data generated by my watch (also a Fenix 3) seems to be in the right ballpark. I still do my own testing separately to find AeT and AnT. The one thing I noted that DID seem to be of help that my Fenix looks at is how recovered I am. I notice that it lines up fairly well with how I actually feel. My understanding is that they calculate recovery based on HR variability. Incidentally, my own AeT and AnT line up pretty well with the Calculation you list as last in terms of accuracy.
I don’t really use HR zones so much and I basically shape all my cardio around AeT and AnT. With those data points, everything else just kind of falls into place for me without fussing with HR Zones.