Do you think Garmin’s Lactate Threshold Measurement is Accurate?

Not particularly, but T pace varies just within a single session, and certainly varies more day-to-day, including based on heat, terrain, and other things you can't control. So unless you own a lactate meter that you use for every rep of every workout, whatever method you go with will involve getting a guess (of more or less accuracy), then trying that for your first few reps and constantly adjusting based on subjective cues.

Adding in chest straps, lab measurements, or lactate meters will make the beginning of that process easier, but it you'll ultimately still have to do a lot of trial and error, and I'm not convinced that using Garmin's estimate is meaningfully much worse. Sure, you'll have some less efficient workouts before you get the hang of things, but you're not a pro, just chill out and enjoy the running.

Too many sub elite runners "overthink" their workouts and the data they're trying to collect. Running isn't meant to be a "gimmicky" sport, there are a ton of variables at play that go unnoticed. If you're wanting to train in specific "Zones" simply purchase a quality HR monitor with chest strap. If you really want to be accurate with the data, go to your local University or College Kinesiology dept and ask to have a max treadmill test which will determine your max heart rate and V02. You can then easily calculate certain paces, thresholds, etc... If you're relatively close to your training HR zones and paces you'll be fine. Running is NOT an exact science, and you'll drive yourself crazy trying to make it so.

The below quote is 100%

FWIW I pay no attention to the LT heart rate, but in me very limited experience as an old slow fat guy who still does some limited workouts, I find the LT pace reasonable - it is a couple seconds per mile faster than current my half marathon race pace, which seems about right.

Old guy (not fat yet). I have the strap...use it when I'm ramping up for a race. On hard workouts, it usually pops updated # on garmin 945. My garmin LT pace and race predictors are all considerably slower, even when I'm not dialed in fitness-wise, despite years of data. I lost any sort of belief in it when I beat my predicted 5k time by 20 seconds while running an 8k and the prediction didn't budge.

I got my LT tested as a local university, it was 25ish seconds faster and 4-5 bpm lower than watch. Figure the lab/treadmill environment is pretty optimal, unlike even the best conditions for a race so I generally have split the difference and found it fairly accurate. Ironically, I did have vo2 tested and lab was exact same as garmin. Go figure.

An issue I have with HR training in general is that so many factors can influence it, such as time of day, hydration/nutrition status, external/internal stresses, etc.

For example, I can run the same pace for my sub-threshold workouts (or even easy runs) 30 minutes after I wake up versus 30 minutes after I finish work and HR will consistently be ~15% higher in the afternoon at the same paces.

Even though HR was quite different on those situations, pace and relative effort were the same. Does that mean my lactate levels would be higher in the afternoon with a higher HR? What very little I read on the topic says not necessarily, but I admittedly know little about the topic.