I decided
Thanks again for all the excellent tips.
This is my decison:
Beijing-Lanzhou by air
Lanzhou-Linxia-Xiahe-Lanzhou by bus
Lanzhou-Xian-Lanzhou by train
Lanzhou-Beijing by air
Lots of Lanzhou in there, but it works as a hub. In past experience, I enjoyed "nothing" cities, so who knows?
The more distant suggestions are excellent. Mostly just a matter of personal preference to not hop around too much by air.
Thanks again, you guys are amazing!