I found Changsha to be easy to root from cuttings.
There is also US-852, a rootstock which is a cross between Changsha and trifoliata. That may be yet another possibility.
I do have one Changsha seedling, growing on its own roots, in zone 8a here in the Pacific Northwest, as an experiment, and can tell you how it does this winter.
Right now it is looking okay, we just had a frost last night.
Jim VH, another member here, had a Changsha seedling growing on its own roots which did not survive, though the grafted version of that same seedling did survive, grew into a fairly big tree and is now producing fruit. He's closer to Portland, and also in 8a.
Obviously zone 8a in the Pacific Northwest is a different climate from zone 8a on the East Coast, so these observations might not carry over as well to your situation.
From reports I have read, Changsha can get pretty vigorous growing in zone 8a on the East Coast, growing on its own roots. Of course, I do not really know how much that carries over into rootstock suitability, or conferring hardiness to scions.
This is just one thought, and it could likely be completely wrong, but I wonder if the problem could perhaps be that the Changsha is too vigorous growing of a variety, and that explains the graft failure rate?
Or perhaps, Changsha, being more drought tolerant, has some adaptation where it more quickly forms protective calluses over cut tissue?
Maybe you should try placing a clear bag over the plant being grafted, to hold in the humidity.
From what you are describing, it sounds like Changsha may have poorer graft compatibility.
This site says "Changsha seedlings can also be used for rootstock on which to bud or graft satsumas"
https://www.plantanswers.com/changsha.htmI really do not know the specific answer to your question, but can at least tell you from what I know, I would guess that Changsha would neither be the best nor the worst variety to use as a rootstock.
I am thinking the real question is will it confer adequate cold hardiness to less hardy varieties which are grafted onto it.
Sorry, I know those are a lot of words and doesn't give you a definitive answer to your question.