I thought this forum might be interested in the accuracy of Garmin lactate threshold metrics. For me it's reasonably close but a little too high. I had done a guided lactate threshold running test on March 11 and my watch calculated LTHR = 180 bpm. Then I did a hard intervals track workout on April 1 and my watch updated to LTHR = 181 bpm. Today I got a real lactate threshold lab test at the UC Davis Sports Medicine center where they took blood samples while I ran on a treadmill and they found that my first lactate threshold (LT1) is 173 bpm. So I'm going to adjust my heart rate training zones down accordingly.
The watch was actually very close on VO2 Max. It shows 49 ml/kg/min and the lab test result was 50.
(I am actually using a Descent Mk2 watch but the fitness features are the same as a Fenix 6.)