I disagree with the japanese needing a defensive weapon. Bu the time Japan had to rely on defensive weapons, she had already lost. The Pacific is not a practical place to defend lol. I would give their best chance at winning the war sans the first few months at either Guadalcanal or Midway. With that, I would go for radar, would have helped save them from the trap at Midway at least.
Also, just so you guys are aware Japan did have a large number of what was basically Panzer tanks, called Chi tanks, ranging from type 1-5. However, they were guzzlers of gas (the thing Japan basically went to war for in the first place) and entirely impractical anywhere but China, hence most of them were committed to the defense of Japan itself.
They also had dual purpose 88mm after encountering German 88’s being fielded by Chinese forces. The Japanese Type 99 88mm antiaircraft gun. But like the medium tanks, it was posted in the home islands more often than not. As far as anti-tank weapons, the japanese used the the Type 93 and the Type 100 flamethrower to great effect as anti-tank weapons, and their own bodies and the Type 99 AT Charge when all else failed.