From my experience with tracking the estimated values of VO2max in various apps there is no benefit. Due to high margin of error any difference shown on a week to week basis has no significance when comparing it to actual measurements.
Heartanalyzer does an okayish job for comparing really long-term trends, but not for the VO2max, but rather their “Cardio Points” = (Exercising HR - Cardiac Exercise Threshold)/(Exercise Duration * Constant)
However, that data is somewhat flawed by the vaguely estimated “Cardiac Exercise Threshold” (similar to a guessed VO2max, I guess) and a “Constant” (how it’s derived is unknown) guesstimated by the app. Still you see a correlation of your workout intensity and duration, which has some value.
But I really like that Heartanalyzer allows to export a monthly PDF with all stats. Especially the line charts for the above mentioned CPs in conjunction with the amount of activity (date, type + average hear rate) is great to get an overview of the amount of training done.
The app Zones does a better job in splitting your data into customizable heart rate zones. If your goal is metabolic conditioning you should spend the necessary times in the necessary heart rate zones according to your training regime. You can compare this conveniently week to week in their stats tab.
Zones also offers a great visualization of METs (metabolic equivalent of task). I find their 90 day and year plot quite valuable to estimate your training intensity relativ to your body weight.
HRV4Training is a staple in my workouts and I recommend it to amateurs and athletes that I coach. It helps to track your heart rate variability on a day to day basis. Personally I’ve bought the overpriced Oura ring for convenience to get data into HRV4Training. I wear the ring only at night. You can however get away with performing a HRV measurement using a Polar HR chest strap every morning for a few minutes immediately after waking.
The ring offers quite solid sleep tracking as well and manages to accurately (as compared to the Watch and the Heartanalyzer app) measure the resting hear rate. The watch regularly shows false rates in the low 30s, whereas the ring’s data points stay in the mid 50s, which is realistic for my training level. (I’ve tried various strap types and levels of tightness, but the false readings remain.)
If you know—judging by the training volume (Heartanalyzer and Zones) and intensity (METs and HR zones in Zones)—that you are doing any reasonably challenging metabolic training and your resting HR is improving (see Health app or HRV4Training or Oura) or staying at a good level then you will quintessentially be improving your VO2max in the long run.
You can use HRV trends to see whether your get into areas of over-training. For me the trends for “sleep score” (given a somewhat regular sleeping schedule and other factors stress at work, alcohol consumption, diet style and traveling) and the amount of “deep sleep”, as well as the “readiness score” provided by the Oura ring/app correlate clearly with a good training level … not too high/low. 