I've had to put my IJA campaign on hold for the time being since I'm trying out Monty's DAS REICH mod for Steel Panthers MBT. I actually started mine in 1931 in Manchuria and found it a very refreshing change of pace. It is very infantry focused as you said, and I think it might take some getting used to to not see periodic upgrades for my infantry
I did enjoy exploring a new order of battle and approaching the game from a fresh perspective, and I fully intend to return to Imperial Japan at some point to get my fill of infantry combat in the jungles of the pacific and asia.
But you're absolutely right, it does get very challenging as things go on. Primarily I think because of the artillery situation and less because of the armor: the Japanese infantry can handle most tanks if they use the terrain correctly, but the Allied infantry can be rough to deal with since Japanese infantry battalions have less or lower quality artillery than a lot of their European counterparts. In the battles I played I felt like I was relying much more on my machineguns and my snipers to fix the enemy so that I could overwhelm them with fire. Tanks (I only bought one company and broke it up amongst the battalion of infantry) were also useful for this but so painfully slow and undergunned that you couldn't expect them to handle everything on their own.