Nxb7 is a very good move, but it can be done latter  I mean, the b pawn is going nowhere and the problem by doing it now is 33... Nc3+ 34.Ka1 Ne4 harassing our f pawn and we ain't gonna be able to defend it because our Rook is already protecting the h pawn and file 1, so we will loose the f pawn if we don't move it now