Whittling those down further makes sense. Then reflecting milestone levels when players spend time would dial it in to give a glimpse of what players experience along the journey.
It does seem like there should be an opportunity to get some baseline metrics, it just doesn't seem like that big of an ask to take a peak at what levels do player play at and at what difficulties.
The numbers may be skewed by factors such as multi-boxing, power-leveling, etc. but some measures could be used to eliminate easy data points such as AE missions and fire farmers. (maybe not)
Without knowing, it's just guessing. Certainty is not required but reality should be a prerequisite.