Bernardo and Stones have contracts through to summer '26, so I cannot see either leaving with a year left. De Bruyne's situation may be dependent on whether we can land Wirtz. If it looks like we cannot and de Bruyne can remain fit and play out the rest of this season, I wouldn't be at all surprised if he's offered a new deal.
It will be Viana's first summer at the club, and if he's anything like Begiristain, I think he'll rely on his knowledge of where he comes from and the international Portuguese market (including those in England/elsewhere) to sign players. In his first summer window, I think, Begiristain brought Naves, Negredo, and Demechelis, all of whom had been playing in Spain, along with Fernandinho and Jovetic. Viana appears to be just as quiet and just as patient, so rather than bring in a host of players, and run the risk of making mistakes, I think he'll be shrewd and look for a similar collection of 4 experienced players and 1 youngster. That would help bring the age down and strike a better balance in what may be another transitional season. It may also be that those he brings in are not necessarily the 100 million players everyone wanted (none of that 2013 harvest were really 'box office') but players who turn out to be the parts of the puzzle we are missing. Of course, such a conservative read would also mean there is no mass exodus.