I usually build the 1 AC and 2 TR on G1 followed by all transports on G2 if I plan to do Sea Lion. No real need for subs as I plan to invade Great Britain.
If I am hedging my bets and may attack Russia around G3 or G4, I build either 1 AC and 2 TR to threaten Sea Lion and use the TR to reinforce Norway and Finland for the attack on Russia later on or 1 AC (to protect the fleet) and 1 DR and 1 SUB to interdict England with. I may build submarines to attack any Allied Invasion Fleet that may form to attack Gibraltar or Normandy.
One game I bought 5 submarines on G1. I bought a few more on G2 and G3. The plan was to recreate the great German Submarine Campaign from history. It did not work out so well. The British player attacked my submarines one by one. If I placed them in a group one destroyer and few planes killed them all. He can only lose one destroyer as submarines cannot hit aircraft. When they were placed alone he killed them alone. I could not scramble to protect any of the submarines. If I attacked his destroyer off the coast of England he could always scramble aircraft to protect the destroyer. After a while I could not keep it up. The first few turns the German can use Luftwaffe to prevent the scrambling by covering the Sea Zone. But in my case, I found I needed the airplanes in Russia and in the Atlantic and I did not have enough. The British player put all of his money in destroyers and aircraft. He built his destroyers in Canada and then sailed them over to attack submarines off England. The Americans killed the other ones in the South Atlantic.
I am not saying it cannot work but I have yet to see the German submarine campaign be decisive. Likewise in the pacific I have seen submarines combined with a surface fleet to destroy the other side. But I have never seen convoy raiding work where the Allies did not already have naval supremacy.