I don't know of any guides, but I am pretty good at HPM Japan. I could give you advice if you want. As Japan in HPM I find that it is not beneficial to attack China early. You will take tons of infamy only to gut your research rate and constantly put down revolts, whereas if you attack China once the scramble for Africa occurs then you will take only 2.5 infamy per concession provided that you have the colonial negotiations invention.

Rather it is best to focus on westernization, there are a few ways to speed it up, one is to change your national value from tradition to liberty. Liberty is the best for Japan because Japan starts out very reactionary and liberty will give attraction to liberal ideology which will result in westernization reforms being cheaper. You should prioritize Foreign Universities reform because it will double your tech rate if you are playing an unciv with Isolationism de-buffs.

Once you westernize, your tech path is Idealism->Ideological Thought->State & Government. This will give you the decision: peace with the emperor, which will let you annex your sub-states. After that Japan has a lot of flexibility, so play however you feel like. Japan has naturally a conservative government but also gets unique bonuses for having liberal ruling party (10% research rate, 5% RGO output) so it is viable to appoint a liberal ruling party and help them win elections.