Imo, it depends on how much you can spend.
In this period of the war, very advanced torpedoes are availalble.
Convoys in general will be heavily protected, so if you want a good torpedo for the initial run, then I'd go for acoustic-seeking torps...to hunt the escorts first.
Late-war acoustics can be used at targets going 10kts or faster...so a salvo with regular electrics (spread over merchants) will alarm them, and make sure they will revv-up their engines.
That is what you want, because they will be escorting a convoy that travels at only around 7-9kts.
So fire a spread over long distance, reload acoustics (2min/tube!), and fire at the escorts.
Dive away, take new position, and from 49m depth (max. launch depth for XXI) launch the most expensive pattern-seekers you could afford back at base.
...and be generous with decoys, when you go to a new position.
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