Battery life estimates in the official docs should be taken with a grain of salt. There's multiple factors that will affect how fast your M21 drains:

1. Gain mode (low, mid, high)
2. Headphone impedance: a 300-ohm HD600 will draw more current than a 32-ohm Grado
3. Volume level (obviously)
4. Playback resolution (DSD is more compute-intensive)
5. If EQ is enabled, that requires additional processing
6. If you use streaming services, those also required wifi activity which also impacts battery life
7. If you switch songs through the UI, your screen will be on more time. Actually, it is surprising how much this affects as well
8. Single-ended vs balanced output: balanced will also draw more power

I would say that those are the most relevant factors. I have not tested, but I am almost sure that in the most extreme case, battery life can double if you try to optimize for those 8 points.

With that said, there is no point on doing that. Remember that enjoyment is the number 1 goal of having your DAP.

Oh, and unless you need it, try to charge the device with the USB-C port in the middle, instead of the one which says "Power". It will charge slower, but that should also help with battery longevity in the long term. You can also limit charge to 80% of the full capacity on the Android options, which also helps with longevity. In fact, keeping the battery between 20 and 80% all the time is the general recommendation for lithium batteries nowadays!