Here is a simple steamroller plan for Japan to go all-in against USSR before USA enters the war:
J1 invade Amur with EVERYTHING available (activates Mongolians in alpha 3 but that’s ok), build major IC in Korea.
Spend everything on tanks (J2) and mech infantry (J3 and J4) built in Korea. Spend on Pacific needs J5 onward (perhaps hunker down in Japan & Korea and wait for European win)
Move the whole horde 1 space westward each turn, with tanks and mech infantry moving 2 spaces per turn to catch up. Bring bombers. If USSR infantry are retreating, chase them. If they stop, kill them.
The 4 waves (infantry/artillery, tanks, mechs, mechs) will catch up at Novosibersk on J6
Moscow falls J8.