I understand why you'd be reluctant to rate him over Joshua or Wilder based on his resume (although Joe Parker is a quality win up there with anything Wilder's achieved), but people rate him highly because of his perceived H2H beastliness and the thought that he could beat either man. He's got that Foreman vibe going on and if I were a betting man I'd put money on him against anyone not named Fury or Usyk right now.
It's not a clear cut thing for either man. You could argue both have chances based on their attributes and past performances. My thoughts are that if Joshua is mentally on top and firing on all cylinders he should be tipped to win on points, but it's a mistake to think Joyce will approach the fight in the same manner as he approached Takam and Hammer, taking big shots to land his own. I think he'll play it a bit safer and attempt to break Joshua down with his jab and mental pressure, trusting to his greater toughness and stamina to turn it up in the later rounds. Joshua in turn will try to keep control for as long as possible and attempt to sweep it on points. I don't think he'd be foolish enough to try to go for a KO on Joyce.