> There were big swings in my resting heart rate whenever I got a new Apple Watch, suggesting the devices may not have been tracking the same way.

First of all, wrist based HR measurements are not reliable. If you feed ChatGPT a ton of HR data that is just plain wrong, expect a bad result. Everyone who wants to track HR reliably should invest in a chest strap. The VO2 Max calculation is heavily based on your pace at a given heart rate. It makes some generalizations on on your running biomechanics. For example, if your "real" lab tested VO2 max stays constant, but you improve your biomechanics / running efficiency, you can run faster at the same effort, and your Apple watch will increase your VO2 Max number.