Rudiger actually offers more in attack than Mendy, and we wouldn't really be changing anything. We already play a back 3 the majority of the time we have the ball anyways, this would result in less shifting around of players. And it's not difficult to switch from 3 to 4 on defence either. This also makes it easier to keep Vinicius on the left
Honestly, Rudiger, Tchoumane and a RWB should be our priorities, aside from Mbappé(and if he doesn't come i think we should rather look for a top attacking playmaker instead, someone like De Bruyne)