Can't know if he has peaked but he is a different physical specimen to others that you have mentioned.
What does not seem to be in doubt is that his confidence has taken a knock and part of that reason is losing his starting place at City to Foden. There have to be other factors too and something has gone on with Pep but not entirely sure what (maybe, and this bit is speculation on my part, Raz needed more of an arm round the shoulder whilst Pep has been tough on him) but Pep did pick him for the UCL final so that looks like a vote of confidence.
I think the issue of whether Sterling stays or goes lies with the player and wether he can cope with being rotated much more than he used to.
What I do firmly believe is that people focus far more, too much, on Raz's failings rather than his attributes. We know that he fluffs his lines more than we'd like but through superb movement, he gets into excellent positions far more than most players. Also, only the other day, Rio F commented on how when he speaks to defenders in the Premier League, they hate playing against Raz because he is a real handful. I think Sterling is under-appreciated by a vast body of supporters but far less by those in the game.
I suspect that it's the mental side of the game that Raz needs more help with and not a case of waning physical powers.
Certainly, I'd move several other players on before Raz but I don't envisage many changes at all this summer.