Thor's arc is very easily spelled out by Frigga: accept the person you are, not the one you're supposed to be. Thor was born and bred to be a king and the mightiest warrior almost like a race horse. He never was in a war he didn't inevitably win, until Thanos. When he tried to rub his victory in the face of Thanos, like he has all his life, half the universe died and there was not a way they could see to fix it. Thor may have beheaded Thanos, but it did nothing to change his defeat. So then he spirals into a massive depression, stops caring about his responsibilities and sinks deeper and deeper. By the time we see him in Endgame, he is so far gone, he cannot even muster contributing in the mission to get the Aether, Then he talks to his mother, who once again teaches him a valuable life lesson which he carries with him for the remainder of the film. But this experience taught Thor he is not right to lead his people since he was an absentee king, so he is going with the Guardians to rediscover himself. Makes perfect, logical sense to me.
As for why didn't he get buff when he called Stormbreaker and Mjolnir, I agree with the writers. Thor is supposed to grow more comfortable in his OWN skin. Whether that be muscular adonis or lebowski Thor, it shouldn't matter. He is who he is. So I like that they didn't magically "fix" his physique. Granted, Thor with a new purpose can maybe train between movies and be in better shape now, but if Thor is still closer to Lebowski Thor, I am okay with that, too.