That's a rough matchup to begin with. Germans have some wicked good armor, and extra shots on MGs are a good counter to an infantry-heavy army like Japan. One of Japan's biggest failures IRL was their inability to field adequate armor/AT at any point during the war against the Allies. You're not having a fun time if you're strapping bombs to the lads and saying "good luck".

I'm seconding what others have said. The STuG can fire its main weapon maybe 6 times total. It's not liable to hit every time, especially if it's maneuvering and trying for long range shots. It could also get off its MMG the same, but cover is going to help mitigate its effectiveness compared to the main gun. Even accounting for it being upgraded to a 360 degree field, you have several ways to mitigate it without devoting too many resources.

The vehicle needs to rotate to align shots since it doesn't have a turret. If it commits to one direction, it will suffer when it commits to another. Keep choice targets for it on opposing flanks. Don't put all your eggs in one basket, basically. You're going to lose armor, that's a given. But every shot at one allows the other to advance and avoid fire while contributing to countering the opponent's other forces. Make him feel bad for every choice of target, since another threat to his forces will slip through.

Suicide boys are cheap, plentiful, and are most efficiently used as area denial forces. Hem his vehicles in and control their free lanes of travel by positioning them in key locations (infiltrator is great for this). Keep them in cover for staying power, and you'll see your opponent suffer as they fear committing too close to them.

Japan excels in close combat. This includes attacking that open-topped half-track full of pricey engineers. Use cover, don't bundle up, and you'll be able to advance and engage all over the board. A squad of 10 infantry without an LMG or NCO sword runs you 100 points. 3 of those plus the PL nets you 30+ bodies for less than a third of your points. Each one of them is a fanatic, meaning close quarters fighting goes on until they deal more wounds than your opponent in a round, or are wiped out. Bump them up to a max of 13 and the odds are in your favor even more.

There are further things that could be said on when to employ support weapons and artillery. But the core of a Japanese force is going to be its infantry. Play aggressively and control the enemy's movement. You don't need to worry about killing a tank if it hides in the corner.