Marko, I'm also planning to go towards Central Asia, but starting from the 1st of September. I want to avoid the +40C super temperatures which plague the region during the summer months. September is still +25 - +30C in the most parts of Kz, Tz and Uz during September. I'm planning to visit those countries + Russia if no war erupts before September.
About Mongolia: There is actually less rain in September
, fall, winter and spring are drier than summer over there.
Southern route (Gob) becomes problematic most likely because that route has river crossings without bridges and the path becomes and stays muddy for longer times when it rains even a little.
Central route should be driveable, no big rivers without bridges. However lower temperatures + moisture = muddy conditions stay longer.
Northern route is colder and wetter, watch Long Way Around and see what kind of hell it is.
You should definitely have a light weight bike without much luggage. Heavy street enduros are a bit difficult to drive there, when I went there 2010 with my V-strom 1000 we had a good weather and not much mud (central route).
For example: a fully loaded BMW 1.2 GS is 60-70kg heavier than the V-Strom, gravity is unforgiving thing. Two guys with Adv. 1.2GS's whom we met in Altai mountains said that they were so badly stuck in sand on southern route that they thought they would die there. I'd say that a 600-700cc bike with a ~150kg dry weight could be the best for covering the distance to get there and make a successful journey through Mongolia.
I was going to send all this as a PM but since I have been a gutless lurker here this system does not let me do that. Must post five messages first.