
Your sentence sounds like "hey buddy, just mimic the Carlsen/Ronaldo strategy and be a great chess/soccer player." At such a high level, where every nuance matters, this is simply unrealistic. Joshua has a different physical condition than Fury and he cannot transform into him.
It was a deeply calculated idea from a former "boxing manager" player who wasted countless hours playing, lol.
Okay, now I understand you

I hope that the managers of Joshua and Usyk are less informed, otherwise we risk seeing two sumo wrestlers in the upcoming duel

Even in the first match, against the background of Usik, he looked like a very slow boxer, what will happen if he gains extra weight? I think that on the contrary, he should lose weight a little and focus on increasing speed - perhaps this will allow him to realize his advantage in strength. Otherwise, Usyk again simply outboxes him in smart boxing.
Again, this is the view of ordinary boxing fan, "the speed," as if AJ was slower. No matter how much weight AJ loses in these few months, his punch won't land if he cannot deal with southpaw style. Why not just rush in-fight with his bigger and more powerful build?
I think one reason is enough - Joshua does not have an iron chin (and some in the world of boxing even call his chin crystal). It's heavyweight box - you just miss one hit and you're out of the game, so rush is a double-edged move, although of course, with Joshua's physical superiority in mind, it makes sense. By the way, this move is also dangerous because if in the first rounds you did not achieve a quick result, then you find yourself in a worse position, since you spent more energy and further prospects look doubtful. It's kind of all-in.