Seems to me that once America was in the war,
the only way to have stopped men and supplies from reaching England and the rest of the ETO
would have been to bomb the shipbuilding facilities.

Herr Schickelgruber needed aircraft carriers and extra long range heavy bombers. Lacking those, the increased tonnage of allied shipping sunk by U-boats was emotionally gratifying to Germans, disturbing to the Allies but largely irrelevant. More shiiping was being built unmolested and with that came counter-measures that soon made U-Boat duty much more hazardous than convoy duty.