I would do pretty much like you did, except I would risk getting further ahead of the convoy, into the shallow waters (even though I also play DiD). Because of the shallow waters, I would fire from greater distance than usual, about 4km, to have more time for escape before the impacts.
As far as the detection ranges are concerned, you can be heard from some 5-6km early in the war, and about 9km late. If you click on the ship's mark on the map, you'll see the circles representing maximum detection range.
If you're dead stop, silent runing, you can still be heard from some 300-500m early, and 1000-2000m late in the war.
Early radar can detect you from 5-6 km.
That was mostly from my experience, and the figures may be off. If I'm wrong, someone please correct me.
