Here are some strategies for Subs
Germany1: You mass the Subs in a single stack, while using Destroyers or Cruisers as shields. For Germany, this would consist of building 1 Cruiser and 3 Subs first turn, use the Luftwaffe to kill the Destroyer in Sea Zone 6, then retreating the Subs to the Baltic Sea. You now have a fleet of 6 Subs and 2 Cruisers, which if attack the British Airforce, will use the Subs to soak up hits (as a result, the British are unlikely to bother. Land the Fighters on Western Europe and Norway. On the second turn, you move the Cruisers to Sea Zones 7 and 3, and the Sub stack to Sea Zone 6, sending additional Subs as fodder if Zones 7 and 3 are occupied (unlikely). The Sub stack can now hit anything around the United Kingdom, with 2 Fighters apiece able to hit Sea Zones 2 and 8, while the British fleet can’t.
Germany2: You mass Subs in the Baltic Sea, though building Bombers would also be useful. Destroy any unit that enters Sea Zone 7, 6, or 3. If it is a small force or a singular unit, send a single Sub along with a Bomber, which should be enough. Doing this ensures that any Allied Invasion fleet will only get to invade once, before they are all sunk. From a technical standpoint, you don’t really need many Subs for this strategy, the only difference between them and Bombers is that Bombers can also be used against Russia, but have half the hit soaking ability. As a result, you would probably build a good mixture of the two.
Japan1: Build a large fleet of Subs, then scatter them across the Pacific, with no more than 1 Sub per space that is within range of a Destroyer. This will force America to mass mainly Destroyers or be overwhelmed, and Destroyers are in turn useless for actually invading Japan or East Asia.
Keep in mind that the purposes of these strategies is primarily to stall invasions by Britain and America, and secondarily to sink their fleets so as to eventually give the option of invading them. So long as that is true, Russia can’t be expected to hold even Eurasia, let alone Africa or Oceania. They have 25 or so IPCs a turn. The Axis will have about 125 IPCs a turn. If they can stall out America and Britain with 75 IPCs, they can easily crush Russia with the remainder.