I think his evolution is way more subtle than it gets credit for. Everyone talks about the 'I love you' scene, which sure, is huge. But the real shift for me was in how he stops performing. Early on, he's a puppet master, scripting every interaction to win. Then he slowly starts to act against his own 'optimal path' because of genuine feeling—like when he gets sick trying to impress her in the culture festival arc. That's not a strategy, that's a person making a messy, human mistake because he cares.
Another layer is vulnerability. Pre-series Miyuki would never, ever admit a weakness. But watch how he starts confessing his anxieties to Kaguya, small ones at first, then bigger ones about his family and future. He learns to be loved for his real, imperfect self, not just his perfect 'Shirogane' persona. The real peak of that evolution isn't the confession; it's the later arcs where he openly talks about failure and fear with her, and they figure things out as a team.
What that all amounts to is a shift from seeing love as a war to be won, to seeing it as a partnership to be built. He stops trying to be her savior or conqueror and starts trying to be her equal and her support. It's less about him proving he's worthy of her and more about them both navigating life together.