I decided to make this a quest, hijacked a type IXB for the extra torpedoes and currently have about 170k tons with a few torpedoes and very little HE left, may still get there.
However, I'm finding this quest boring and very contrived. It just doesn't feel right attacking a convoy, surgically removing the tankers and letting everything else go. Same with singles, chasing down contacts and then letting them go 'cause they don't displace enough is not war and not in the spirit of SH3. Out of sheer boredom, I've jeopardised the quest by wasting torpedoes and HE on the occasional C3 or small tanker, 'cause nothing heavier came along for a long time.
Cherry-picking is the only way to get there and without the luck of finding a few strawberries (liners, battleships or carriers), this quest is really a grind. I'll persist, but won't do it again.