My mind keeps circling back to Robin Hobb's 'Assassin's Apprentice' series. That isn't a hero who just gets stronger; he's literally broken down, rebuilt, and spends his whole life grappling with the consequences of his choices and the weight of duty versus personal desire. It's a profoundly painful read sometimes because the transformation is so earned and never feels like a power-up. The entire Realm of the Elderlings saga is just a masterclass in writing a person, not a plot device.
For a different flavor, Lois McMaster Bujold's 'The Curse of Chalion' shows an older, weary hero whose growth is about healing from trauma and finding a way to believe in goodness again. The magic system is tied to the divine, but the real magic is in how Cazaril's quiet, stubborn decency changes everyone around him. It's a much more interior, spiritual transformation than swinging a bigger sword.