Usyk already has fought 12 rounds against larger HW'S. Did you forget that in both Joshua fights he dominated the last 4 rounds. Difference between now and before is that Joyce is really huge. Doesn't even look like the same man. His advantage vs Usyk is his iron jaw. Only way for Usyk to win is to outbox him; no way does he get a KO.
Joshua doesn’t fight at the tempo that Joyce does, neither is he a constant pressure fighter finally he doesn’t have the power and consistent use in his jab. Fighting Joshua and fighting Joyce are completely different tasks, their similarities end at their height, Joyce is naturally 30lbs heavier than Joshua yet retains superb stamina and energy levels.
Usyk wins this fairly easily and it wouldn't be that exciting. Joyce's technique is crap and he's slow as hell.