Theres a lot of approaches with Japan that can work, but it is really difficult due to how many different places the allies can atack you.
I personally look to declare on J2, unless waiting a turn can really benefit the europe side, mostly based on usa1 purchases and what india and anzac did with their trannies.
I like to buy IC and 2 trannies on J1, and I feel that this factory cranking out mechs means that the ground units already on Asia are more than enough to push into India and force its capture, or the India units to run away to the middle east, usually on about turn 6 or 7.
This frees up my navy to aggressively stack against the combined Allied fleet, helping to hold the dutch money islands without contsantly trading them back and forth with the allies. So far my favorite spot is to stack and hold off of Java with several trannies and ground loads, with enough air and maybe ground on phil so the allies cant really go get it back easily. Ive seen other spots played and work very effectively tho.
This leaves contsant threat on India, while giving my ground factories (usually i put a second one on FIC once I take it) The time they need to move in and stack burma without India being able to take it back.