It's good to have a strong group of homegrown players who know each other. But besides that, Walker and Rose are very good fullbacks. I'm not really sure whether any of the mentioned full backs like Mendy, Gaya, Bernat, Semedo, Sidibe etc. have anything over those two but they'll cost about the same. We're not getting a player like Carvajal from Real Madrid or Alaba or even Sandro from Juventus (are we?). Even Bayern Munich are said to be interested in Walker, he's a solid right back with good pace and defensive skills.
As for Pickford, I have my doubts over Ederson who will cost twice as much money and I don't know whether he'll be good or will end up like Bravo who was also very good at Barcelona but so far flopped in the Premier League and cost us a lot of points, but that's just me. I don't watch Benfica regularly.