As Japan I would attack Russia for a few reasons:
1. J1 attack on UK, ANZAC, and USA will hurt other Axis in Global so Japan shouldn’t attack on J1 or even J2 (depending on situation). Other than China and French Indo China, Japan needs other sources of IPC and that would be Russia.
2. by going on the offensive against Russia, Japan can bring to bear the full force of their airforce and even navy. even facing a stack of 18 infantry and maybe one fighter, japan can destroy this force with about 10 land units and most of their planes.
3. by eliminating this stack (or two smaller stacks), Russia will be empty in the east and Japan can then turn most of their forces south and reposition for a J3 attack.