This could be compelling arc, if it didn't go around circles for 4 games

You're thinking that way because you're in the mind that "his problems are only locked in his backstory, and cold steel should have solved them". From what i've experienced, what Rean went throught during his character arc(talking only about cold steel) was even worse than his backstory, which is completely different from the protagonists before. Even Kevin,who had by far the darkest backtory of them all, sky 3rd did was basically solving it. What cold steel did to Rean however is instead of resolving his issues, getting deeper and adding more into them which is not the same at all( Rean was at his best mentally in CS1)

Right now i think that him and Kevin are far more interesting than the other 2 .But i'm gonna wait until Reverie however(yeah did not play it yet) to have a better assessment on the topic