It's an interesting idea. On the one hand, it seems unlikely that Japan would adopt a non-British style of government, what with the very heavy British influence there since the 1870s (someone had to protect Japan from the Americans, and Japanese culture is very similar to British in many ways). It's certainly possible, though - one interesting consequence might be Japan trying to solve its resource shortage through invasion, rather than vassal states and trade links, which would do interesting things to the balance of power.
Well to be fair Japan did plenty of aggressive invasions early on, though in hindsight they served an actual military strategy that they learned from the British i.e. do not let foreign powers control nearby islands that can be used to attack the Home Islands. Formosa and the Senkakus were taken during the Sino-Japanese War of 1895-1896, Karafuto and Kuril during the Russo-Japanese War of 1904-1905, and then the North Pacific Islands in WWI. And as previously-mentioned, divide et impera was heavily-championed by the Japanese in China during and in the immediate aftermath of the Eight Powers Intervention...on the same (though never openly-stated) reasoning as the British were against any single nation dominating Europe. Granted, the British and the Russians were also heavy advocates of the policy, but it was the Japanese who first openly advanced it with regard to China in the 1901 Brussels Conference which ultimately divided China.
Aside from anything else, the American public would go berserk at an attack on American soil, like they did in the 1950s (and Luzon wasn't even an American state). Public opinion was already very anti-Japanese, at first, when it was them making the unprovoked attack - the rallies in favour of annexing the Ryukyus seem utterly laughable now, but there was a large strand of the American public that believed themselves to be invincible on the global stage.
Strategically-speaking the Americans were invincible, at least prior to the age of ICBMs and other long-range weapons. Their industrial heartland was beyond reach after all. And the North Pacific Campaign was a close thing as it was. Yes, the Japanese destroyed the American Pacific Fleet in that campaign, but it gutted their naval aviation and greatly-depleted the IJAAF's reserves. I'm not sure if Japan could have stood up to a second major operation like in the North Pacific, at least not without a minimum of a year to replenish their aircrew and machines.
One thing I wonder is what kind of international reputation a militarist Japan would have? Sounds silly, but consider...
OTL, Japan's major wars were the two Russian Wars and the Pacific War. In the first and the last, they gained a reputation as relatively chivalrous in war: they adhered to standard practices regarding POWs, etc. In the Second Russian War, though...there's some awful stuff happened on the Siberian Front. A lot of it gets glossed over because, quite frankly, the Reds did some awful things too (the million plus Jews and others who died in the Communists' gulags) and many of the officers responsible got quietly sidelined after the war - quite a few were politically radical. And of course, success breeds silence - today's Far Eastern Republic is a very democratic nation and a member of ATO in good standing, so of course nobody talks about what was done in the course of bringing down the Reds.
But in a militarist Japan, such officers would probably get promotion, not sidelining. Add to that more aggressive military action...might Japan's reputation be less than ideal in this world?
OOC: I'm acting under the assumption that a less militarist Japan means - by and large - better treatment of POWs ITTL and adherence to the rules of war.
Well, it's kinda hard to see the Japanese military being undisciplined, I mean as far back as the Sino-Japanese War their officer corps was known to be very draconian and obsessed (even by Prussian standards) with 'iron' discipline. Summary executions were common for troops who went on rampage during the war, and even high rank (either military or noble) did not excuse one (at least not completely high-ranking or noble officers could get away with dishonorable discharge instead of execution) from punishment. Even though while not as draconian as they used to be, Japanese military discipline today still has plenty of Westerners raising eyebrows.
It's honestly surprising that some Japanese troops went on rampage during the Pacific War, but they paid for it, both during and after the war. IIRC there was an instance - I can't remember where though - where around 2000 troops or so from several regiments were tried and publicly executed by Field Marshal Yamashita to reinforce discipline, and as you mentioned there were the numerous reassigments and 'early retirements' after the war.
OOC: IMO it would take nothing less than NKVD levels of disciplinary action to keep the IJA on the straight and narrow.
Maybe...you have to wonder why they'd bother, though. The DEI and Indochina were chafing at the colonial leash already. OTL, Japan just stayed out of the whole thing and slipped them arms under the table. Fast-forward to the 1960s, both are finally independent...and then they joined up with Japan, Korea and the FER.
That, and the deal with Britain and Australia wherein the former guaranteed the Malaysian Dominion's sovereign rights and the latter would keep Papua New Guinea in exchange for staying neutral in Indonesia.