help his team defeat an evil dragon, proving that it doesn't matter what occupation you have, as long as it towards helping others
dies saving his friend, but was rewarded a chance to return to life
MC: "Okay, but changed my occupation so that I can use magic and not be useless!"
...Fucking really?
he didn't say he wanted to "not be useless", he didn't want to be labelled as useless. Societal stigma do be a real thing, and honestly pretty believable that mana would become a discriminatory factor in a world with adventurers and monsters.
Of course, this is understanding that those factors are a choice by the author, not something that's always inherent in fantasy worlds (we got levels, anything goes honestly).
At least they completed their mission as a party, so there's no looming pressure on him to live the life he wanted (he probably was envious of people with mana because as we see with the crystal, lack of mana does actually mess with his day to day life).
My main question I have at this point is: how old are they? like, he was a child when he got the motivation to train, but you'd think there'd be some older people around to help with this supposed world saving mission they were doing. Also, why have him become younger? wouldn't that mess with his relationship with his old friends? like, he jsut wanted to be a sage, why'd the goddess have to shotafy him too