Ok. Obviously you see it from the Jap point of view. However - and it's a statement from another expert (castortroy, see above)-
As the Japanese, consensus seems not to go over level 6.
I assume going beyond lev6 could entirely depend on your supply situation. Or did I overlooked something ?
My supply situation is okay. Some of my HI bases are low but that is because the in game RR system is not very finely coded and does not do the greatest job or moving supply and fuel around.
China and Korea have a lot of supply, more than they need, part of your job in the scenario is slipping supply past Allied blockades and back to the HI.
As the Allies having to go up against a lvl 7 or 8 fort stacked with 20,000 IJA troops is pretty disheartening.
Playing as Japan as I said, there is not much you can do in the scenario. Building forts seems like a pretty good use of supply.
don't know what date you are talking about, but even the best players I know seem to run into supply problems in late stages, the better you play, the more troops/aircraft/pilots and the more supplies will be eaten.
And I can asure you, 20,000 troops would never dishearten me, not even if they would sit behind level 9 forts. That's just one division with some support, usually just close to 400 av. By the time I have to deal with Japanese level 8 forts (usually not before later stages) I am able to overrun bases with 20,000 troops easily. 200,000 is a different story, but 20,000 I wouldn't really care about forts.
Not to be forgotten, good defensive terrain gives you more than forts.