If they fight, Fury will overpower Usyk just as he did with Wilder by being on the front foot and getting Wilder to walk back. I think you are spot on when you say speed will not help Usyk because when Fury hits him he will feel the impact. Not many would give Usyk any chance of beating Usyk but before that fight can take place he needs to defeat Joshua in the re-match and also Fury must defeat Wilder in their re-match.
I still see Fury being the one who walks away from the as the unified undisputed heavyweight champion before the end of 2022. Maybe a fight or two in 2023 before he retires.
There's only so much weight someone can put on before it becomes a hinderance as bodybuilders often don't make good fighters and are too bulky. Usyk obviously isn't a true heavyweight and I think he would struggle to get much bigger than he is right now but he should just do what he did against AJ and try outbox him and dodge whatever Fury throws at him. Easier said than done though and I think Fury is a better boxer than AJ.
Indeed Usyk is not a heavyweight. Joshua weight was 8kg more than Oleksanders. And Tyson Fury is (or was) 16kg heavier than Joshua.
Btw, comparing Fury and Joshua, Fury is more faster. I think this is due to Anthony being a heavyweight because he has pumped his muscles, which hinder more than help. While Fury has a lot of weight in his "life buoy ring". But frankly, Fury is to big for Usyk. Speed wont help him to avoid Fury's bombs.