Sources such as Basil Collier's "The Second World War: A Military History" list the Japanese forces during the war as being 6 million. My understanding is that the background of these troops is roughly as follows:
Of these 6 million troops, 2 million started the war (1937-41) in China, where they became veterans fighting the Chinese.
After Pearl Harbor, these experienced troops were mostly shipped out to conquer the ASEAN countries (Burma, Philippines, East Indies, etc.), or to defend the Pacific Islands (Saipan, Rabaul, Tarawa, Iwo Jima) against American attack.
Two million formerly "garrison" troops left Japan for China in 1942-43, where they acquired "some" experience in the later stages of the war. These were replaced by a new batch of "garrison" troops with little or no experience.
Is it true that the Allies had "already" fought the most experienced Japanese soldiers on their way across the Pacific, and in the ASEAN countries, and would have been fighting relative "greenhorns" in Japan? Just as they fought Japan's best pilots early in the war and "Kamikaze" caliber fliers in the end?
If you look at the Battle of Okinawa, you'll see that these guys were still well able to dish it out, even this late in the war. This is not the Volksturm in Germany late 44 and 45.
A determinant factor was largely whether the local Japanese commander in charge would be dumb - suicide charges - or clever - anything but suicide charges. At Okinawa, one officer (Yahara) was recommending attrition and harassment, another officer (Cho) glorious and fearless attacks. The Japanese did fairly well, i.e. did not collapse, until the first lost the argument and they reverted to banzai mode. Yahara, IIRC, wrote a book about the mess afterwards.
Same thing at Iwo Jima, a bit earlier, with a commander unwilling to waste his men, but willing to sacrifice them to inflict maximum US casualties.
The garrison at Iwo Jima was typical "2nd raters" as per this logic as this unit was reformed in May 1944 from what seems to be non-veterans. Basically, while this is an interesting question, I think that what might be a massive factor with green Western troops - morale and breaking or not under fire, just did not apply as much to Japanese troops, who would not break but were outgunned and often wasted by their officers.
China veterans may been less willing to forego heroism, believing elan would win the day - which it might do against more limited Chinese firepower. Or they may have been more clever and cautious instead. It is hard to know and one would expect variation from individual commanders, along with extra resistance due to invasion of the homeland.
The comparison with pilots can be a bit misleading. First the Zeros started out technically superior, with some flaws, but were gradually just plain outclassed. Second, the Japanese Navy apparently was apparently not good at mass training. They had excellent pilots at the start, but once they were killed, no one of the same caliber replaced them. Infantry just requires a lot less training than pilots.
The civilian casualties in Okinawa were horrific btw.
Due to lack of resources, Japan was not able to resupply their Navy or Air Force adequately. Many historical buildings and statues were torn down deliberately from scrap metal. They were not able to repair their planes and also had a shortage of fuel (this is the reason for annexation of South East Asian territories). This resource issue is the main reason for “Kamikaze” tactics in the air.
They still had a large standing army and trained new soldiers throughout the war, having a large population to draw from. Japan had military service from 1873 and in the years 1943-1945 this was expanded to men under 20 and “subjects” in occupied Taiwan and Korea. Heavy casualties sustained by the US in the Battle for Iwo Jima and Battle for Okinawa. Japan still had skilled military commanders and large population to draw troops from.
When the USSR invaded Japanese occupied territories in Manchuria during the final stages of the war, these fell quickly as they were lightly defended. It is likely that some of the “experienced” troops here were redeployed by this stage.
Considering the heavy casualties in earlier battles, it was actually thought that Operation Downfall (a proposed invasion of the Japanese archipelago starting with Kyushu) would be even more costly. Once islands closer to Japan we're taken, these were used as a staging area and most major cities were heavily bombed from November 1944 until August 1945. Yet Japan refused unconditional surrender and soldiers fought to the death rather than be captured. It was thought that the Japanese would actually present more of a challenge when defending their “home” islands. This was part of the justification for the proposal to use tactical nuclear weapons to strike the cities of Hiroshima and Kokura instead.