Garrisons. I annex the lot and only eventually puppet the westernmost warlord, usually on sept 2nd 1939! I will make a few 2 brigade garrison divisions and place on a few key ports, VPs or airbases. The rest gets policed by cavalry units who rush around squashing any uprisings.
Gaming. I move my leaders about and try to max up their leadership. Get Yamashita up to level 7 and he’s then set as my spearhead Army Group commander with terrific supply savings. Also Nishio is a good logistic wizard to try and max up. I split my air units into individual units to give every available leader some exp ( Ozawa). My air experience gets to about 40 and my ability to research those doctrines skyrockets. My oldest navy units never leave Tokyo. I make a few battleship bombardment units to catch the enemy navy and support the army. I put my best admirals (spotters) in charge of small groups set on convoy attacks, purely for the exp gains and the naval combat exp level. Leave some Chinese ports open, just for the convoy raiding sport. I have a chosen admiral in charge of a carrier group. They find out which port the Chinese navy is in and wait outside to catch it ( sometimes my army chases it out). They also send their CAGs to bomb the enemy army. This gains exp in many ways, but also gives the admiral a slow boost too.
After that. At some point I will take out Thailand before war with the west. I think I could have it as an easy puppet after the French collapse and an invasion increases my threat and will mean the USA hate me. I intend to take Singapore and Burma. If I own Thailand I can build in it, and I like to have airbases (each with extra infra) down the neck of it. I intend to have radar listening posts in Thailand too. This enables me to watch for and chase enemy subs coming through the straits. A large airbase and radar station up there in northern Thailand will give me an overview west towards Rangoon, so I can watch what is coming landward or seaward from that direction.
The moment I have radar available, I obviously research all those naval radar and ASW techs. I’ve used electric theory research to speed radar and these others up ( those DD and CL practicals help too). Make sure you start aa upgrades early enough that your best ships (CV/CL groups) are fit for carrier engagments by the time you want the next war to start.
When I decide to war on the western powers, I go all out. USA has embargoed me anyway. Take out Phillipines, Hong Kong and Indochina (another strategic effect) and then Java, Singapore, followed by all remaining Dutch VPs. Germany finishes the Netherlands VP total and they surrender. I have the DEI as a wargoal, but in order for the dutch surrender to fire properly I had to momentarily “tag” switch to GER sometime before their fall and make sure that Germany had the “conquer” wargoal set ( I think that’s now substantially different in TFH?) I’m now set for oil and rares. I take Guam and Wake early. I don’t garrison any islands because it means convoys (sub attacks!). I pause in Malaya/Burma and rest/repair while some units head south and puppet Australia and NZ. I’m pretty much using one full Army on these offensives while having the other in reserve in Tokyo. My reserve army includes a few light armour, armoured cars and motorised units that spent the China war sitting still out of fuel in various places. Older reserve fleet there too, intent on saving fuel.
Enemy subs are biggest pain. Search them out and don’t forget to port strike them whenever you can. I use my few subs well ahead on convoy raids and also keep an eye on what is in enemy ports. I switch to auto create convoys for a short while myself, then player control and turn off any going to stupid or vulnerable places. Those spare divebombers are on naval strikes around key ports looking for subs. Spare CAGs are resting so that they can replace worn out groups on my carriers.