Japan’s land army was no match for the Soviets. Even being stretched thin in the west, it likely wouldn’t have taken much commitment to severely limit japan’s gains in the East.

I suspect it’s quite likely that as Japan decimated its own resources trying to invade Russia, Roosevelt would have eventually found a casus belli and started a war in the pacific on his own terms. The fear of this exact thing is why Japan attacked Pearl Harbor in the first place.