You don’t need a watch. You can estimate VO2 max yourself by doing a max effort 1.5 mile run and applying a regression equation: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4795745/

That’s all the watches are doing anyway. Even if you do buy a watch, keep in mind you need to run regular max effort runs to get an accurate value.

So if you buy a watch but run slower than max effort, wouldn't it the watch record an inaccurate value? The watch probably only determines effort through heart rate but that has significant individual differences.

Very helpful, thank you