I agree with most of your points. However, one thing that I would like to note is how the steppe-conveyor-belt will function ITTL, with the HGE forming what's effectively a giant plug at the western end. I'm certain some groups would break through, only to be assimilated into the larger Hunno-Gothic polity, but I think that far more will be halted and kept further east than IOTL. I think the end result is probably much of OTL's Turkic migration (think the Bulgars, Hungarian, Pechenegs and Kipchaks) being redirected southward into Khorasan/Persia/the Middle East at large, and some of them will doubtless go eastward into India. I suspect this mass movement will depopulate much of Khorasan and lead to an earlier and more extensive Turkification of the region. There is more extensive stuff I will fill in later, when I have time. Thoughts?
My thinking is as follows: in OTL, the Turkic invasions of Central Asia and beyond were so thorough and persistent
because China existed.
Successfully invading China (as opposed to staging temporary incusions, naturally!) is not exactly easy. Quite the opposite. Naturally, it's attractive, but it's the kind of "very high effort for a very big prize" kind of deal, which is only for the big boys to contest. So that's why the heirs of Genghis Khan -- most successful conqueror ever -- were the ones to pull it off.
Anyway, with China just
being there, and generally beating back all attempts at over-running it, the steppe peoples could only "drain" in two other directions. Whenever that great, grassy... uh...
couldron... boiled over, the critical mass of horse-riders would surge into the Eastern edge of Europe, or into the Northern edge of Persia. And they did both of those things, with alarming frequency.
If Europe becomes a "Western China", then this "drain" is indeed plugged. My point is: the shattering of China means that the drain on the other end
isn't plugged anymore. But much of the wealth of China's civilisation (and the value of its land) will still remain. so my thinking is that the horse-riders who are now barred from surging into Europe won't just go into Persia. They'll go into the chaos that is Northern China.
That's why the "revolving door of warlords" that you have imagined there is so plausible. Every time they can even start
thinking about stabilising the region, a
new wave of horse-riding lunatics crashes the party.
So, on average, I'd expect the number of steppe invasions of Persia to remain roughly equal to OTL. Those of Europe would
decrease, and those of China would
increase. That's how I view the dynamics at play here, at least.
A potential way to avert this is something I have once suggested for a scenario: have the North of China indeed consolidate again, and thus able to keep the steppe invaders out. Of course, this would then force Southern China to
also consolidate, to avoid being conquered by the North. Outcome: two "Chinas", culturally distinct from each other. One "half Mongol", the other ultimately "half Vietnamese". But to be honest, I think that "bi-polar" set-up would rob your scenario of that charming facet; a multi-state political order in Southern China, which evokes the political situation of Western Europe even as ATL Europe evokes OTL China.