Forgive me for sounding like a broken record, but Ryukyu is even more retardedly difficult. Ulm is Catholic, has Western tech and is part of the HRE. Ryukyu aka Okinawa is Animist, Chinese tech and shares no borders with anyone, so the only way to start conquering is gloriously suicidal no-CB wars. Admittedly easier since Ming got balkanised.

Fun though.