How much height of a mountain or any topographic high, decreases for a million year averagely? What is the range? (min / max)
This may not answer your question, but it indicates that erosion is extreme at high altitudes.
More than 60 percent of the sediment delivered to the world's oceans in the prehuman world originated from erosion in mountainous areas with elevations greater than 3 km above sea level.
Muddy Waters - More sediment is entering rivers, but less makes its way to the sea - Sid Perkins
Only about 1% of Earth's area is above 3 km.
Although erosion rates vary widely depending on the location, tectonic setting, climatic history, fluvial incision, human disturbance, and other factors, there is a paper available online that illustrates and discusses the global variation in erosion rates as measured using cosmogenic nuclides. It is:
Portenga, E.W. and Bierman, P.R., 2011. Understanding Earth’s eroding surface with 10 Be. GSA today, 21(8), pp.4-10. https://www.geosociety.org/gsatoday/archive/21/8/article/i1052-5173-21-8-4.htm https://www.geosociety.org/gsatoday/archive/21/8/pdf/i1052-5173-21-8-4.pdf https://www.geosociety.org/datarepository/2011/2011216.pdf
In addition, An introduction to erosion rates and the processes that control erosion rates in mountains is the book: Burbank, D.W. and Anderson, R.S., 2009. Tectonic geomorphology. John Wiley & Sons.