Sooo, that thing that I was upset with Ming over happened. They sat in a war with Korea for a long time, and as soon as Korea started taking back provinces Zhou declared war and Ming peaced with them and took back some land. That was nice, but what wasn't nice is that I ran through all of my money, all of my manpower, had tons of rebels freaking out due to war exhaustion being ridiculously high because you can't get out of a coalition war (at least change that so you can get out if your WE is high enough? Please Paradox?), so I decided to just abandon that game and start a new one as one of the vassals and see how that works.
For one, I was wrong. They don't start out at higher tech levels, they start out at the same tech level but some of them have better leaders and they can tech up better like that. So, instead of being 1562 Japan with two vassals left, I was able to use Uesugi to conquest Japan completely by 1472. The problem with this game is that Ming has absorbed Korea and doesn't like me because I took everything in Japan by force. I do like how Aggressive Expansion or w/e is different from EU3's Infamy, though, in that it has an area of effect and people care differently about it. Like, by the end I had -400 relations with my Japanese neighbors due to AE, but Ming and Korea are closer to -100. That's probably just due to the annexation of the Western vassals of Japan and Japan itself, and the nearness of Ming for the end of that since they owned Korea.
I'm pretty sure the best bet is to still use Japan, but diplo-annex the biggest vassals and use events or fabricated claims to get all the small ones. Still take it slow, but that way you won't build up ridiculous levels of AE. That's probably obvious, but I was just checking out the different ways stuff works. Probably going to do another playthrough now that I know how the game works a bit better and see how fast I can unite Japan. Also because Ming is super gigantic and doesn't like me, and it took a bajillion admin points to core everything and keep myself stable long enough not to collapse completely so I'm not Colonial yet and I think I was by now as the Japan playthrough.
After this I'll probably try going back to my EU3 roots and play colonial Teutonic Order.