Are you playing U-Boat or fleet boat?
With either, long range shots at a convoy like that with no follow-up are pretty easy pickings, so long as you hit the target. Even going deep isn't necessary, just turn tail and run away as soon as the shots leave the tube.
This is pretty authentic. In real life the captain would be cut off at the knees for not following up on the same convoy. It's fairly safe to do that as long as you turn tail as above, do and end-around to the other side of the convoy and repeat the procedure. I usually close to under 2,000 yards with TMO and love to get on the unescorted side of the convoy, get in under 500 yards and conserve torpedoes.
Actually I like your strategy. Try it with TMO if you need more danger.
