Maybe an unpopular opinion, but I think it's always a good idea to renew the squad after an extremely successful season, especially with players of certain age/or players that will not renew. We should sell Mahrez if there is a good offer. He is important, but he is not a starter, and due to his age, it's unlikely he will be a starter again. 40m for him, we would be crazy not to take him.
And then, Gundogan or Bernado. Here age is irrelevant, because Bernardo WILL leave in 2 years for certain (he wants to play for Benfica while he is close to his prime). So right now, it's a choice of whether we want Gundogan for 2 years and get 60-70m for Bernardo, or we want Bernardo for 2 years and Gundogan leaves for free. The 3rd option is having both, but that would still mean to replace both in 2 years (and maybe even KDB), which I don't think is wise. For me, I have zero doubts is time to take money for Bernardo, and slowly rebuild our midfield.