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 this as the unified undisputed heavyweight champion before the end of 2022. Maybe a fight or two in 2023 before he retires.
I agree which is why I think that once Usyk gets rid of any obligation he has left with Joshua then he is going to avoid the fight with Fury for at least a year, he needs to get heavier and he needs to do this by more natural means than the path Joshua did which was to pump his muscles, he needs to gain weight in a more natural way, he does not need to reach the same weight as Fury but he needs at least 20 more pounds just to get a chance against the unification fight we are all expecting.