So long as you can invade Korea and exploit its resources, that's a start. The Chinese should get around to fighting.
The Korean peninsula has Chinese flags in each province (as its a "protectorate") and I can't seem to invade because despite being at war with Korea these provinces are apparently "neutral" grrrrrrr
You need a CB against China, the way China would need a CB to declare war on you, to get Okinawa (Ryu Kyu). Their "protectorate" is their property.
As you get more objectives in China, youll eventually get CBs against Russia and China every few years until the 1870s (which is the more reasonable period to declare war on China, if not even later).
Id concentrate on getting the troop techs you need, declaring war on Annam, Aceh, or Rangoon, and fighting there during the 1860s to build up troop experience and personal experience as to how supply transit is going to work as well as supporting armies across the sea and dealing with weather.
Declaring war willy nilly against China is going to get millions of Japanese soldiers killed while you figure out you cant build ports in territories you arent claiming, that rain storms will destroy your cohesion, monsoon season is your worst enemy, and snow is going to make your soldiers cry frozen tears.
Also gives you a chance to figure out the convuluted MTB, merchant ship, transport, supply fiasco, before youre at war with their billions of war junks.
"Hey! 'said my name is called 'disturbance', I'll shout and scream, I'll kill the king, I'll rail at all his servants!" -Rolling Stones 'Street Fighting Man'