Thanks for your wise words!
As it happened, after some thought I decided to tough it out (rather than restart looking for tributary status). Peaced out Korea and Haixi (managed a humiliation) while successfully running away from Ming stacks, everyone got away. Then just trashed and blockaded Ming at sea for years. Lost three ships (captured more), killed over 60 (the Hosokawa galley bonus plus my paranoid heavies plus lucked out with 6-shock and 4-shock explorer-admirals for a while), they kept rebuilding! Eventually I was able to peace out for Formosa and a nice wad of money so presumably they were having mandate issues (how can you tell?). Meanwhile I'd annexed all Ming's Philippine tributaries in separate wars. Now while I have truces, I'm cleaning up the remaining 9 provinces in Japan owned by daimyos.
Second round with Ming I'll have a fort finished in Formosa to match the one already in Sakhalin, to tempt the AI into dividing its forces. I would have taken out Korea separately, or at least grab their two mountain forts, hopefully with time to fortify the coastal mountain province conveniently adjacent to both. But I won't have time with only a year between the two truces ending. So I'll take up your suggestion of declaring on Korea just before peacing out Ming (unless they come at me anyway!). After that the plan is to stand and selectively fight Ming in the mountains until Ming breaks, and otherwise try to build up manpower. Money no longer a problem!
1517, taken Exploration (full) and Admin (just 2), Tech up to speed and colonialism embraced. Because of Ming I think I'll be different and take a military idea group when tech 10 rolls around in 1518, which also will take the pressure off admin. Quailty seems optimal given the 4th and 5th Hosakawa ideas will arrive at the same time (+20 force limit, +20 manpower).