What’s the best way to accurately measure one’s VO2 max, or at least ballpark it sufficiently so that you can track improvement over time without it being lost in measurement noise?
As someone else mentioned, a classic poor man's test of VO2 max is simply to time trial a set amount, such as seeing how far you can run in 10 minutes, and use a calculator. So the same thing with extra steps.
If it's meaningfully improving from year to year, you're improving all the things you care about.
(The best way to improve that is to slowly increase your volume of cardiovascular training over time, with at least 80% being zone 2 work.)