My priority for you would be CB > CM > Sancho.
Sancho is a great player but its obvious when you watch him that you will need to play him on the left to get the best out of him in the Premier League. His skillset makes him excellent in transition and broken field situations on either side but against settled defenses and low blocks he is very useful on the left but much less effective on the right, which isn't really a knock on him its true of almost every right-footed wing player in today's game (same could be said for Gnabry, Pulisic, Coman, Sterling, etc). The problem is that there are more transition and broken field situations in the Bundesliga than the Premier League and United is already very good in those situations.
United would be better off buying truly top level CB and CM, then spending less on a left footed creator like Ziyech or Bernardo Silva (both apparently looking for new clubs) to fill RW for a couple years until Diallo is ready.