IX-D2 and XXI are really two different beasts, I suggest you to still train two torpedo men. The reason for that is because, SS vs SS is what KM is best at, handicap your self with only the four frontal torpedo from SS1~5 is not worth it.
Type-XXI, even though I never played one, I can see its niche, with all six of the tube focus in the front while equipped with the fastest and the most deadly proxy torps. XXI serves the sole purpose of laying waste to high tier BBs and any subs that dare to face it head on. But a type XXI that got ambushed from behind is pretty much a dead XXI.
Whether or not you want to be a traditional KM sub player with the IX-D2 or try out a hybrid of IJN and KM like the XXI, is completely up to you.