You just don't understand it and getting ridiculous.
The Game has a problem with mercs and money and Ming utilize it. As I said a lot of times now.
Defeating it is not Problem, if you use Mercanaries like they do. Other wise you won't carpet siege them before you run out of manpower, because they spam mercs(like all major powers. In my game it would be Rev. France, Ottomans, Ming, PLC, Scandinavia, GB(all have 0 manpower all the time and attack other nation with tons of mercs).
I never player a horde, so I don't know where you come from. Since MOH I mostly play Japan have as much Development as Ming only restricted on the Japan region, Before they mercplosion I like to vassalize Korea. I never wanted and tried a World conquest, because I personally don't like it. I play the game to have fun and mostly not for achievements.
I also like that Ming is quite strong, but I also think it should decline if pressured enough, I also think that with every Empire that is too large, AI or player made should crumble easier. It is now so ridiculous easy make giantic Empires.
Low unrest, autonomy and easily accepting cultures and weak Rebels are a problem.
It would be nice if Quing would take over, that would be very nice to see, but Manchu tribes seize to exit in the first 100 years, because of Korea or other neighbors.
That's the reason I didn't want to debuff the Mandat to much(for a strong Quing), but some things must be changed if the merc spam/money is not solved.
I think you getting unreasonable, because you want this game to have an "endboss".
So you are telling me everything is fine with those Nation? With the wealth and Mercanaries around?
I proposed my Solution. If you want Ming to be your big endboss propose a Sollution.