Of course, it’s wanting to protect Usagi that truly unlocks Ami’s power. Ami was powerful already, but to truly focus it she needed a connection to someone and someone who made her feel valuable as a person. Usagi was that person who allowed Ami to feel proud of herself and see that people can like her as a person and value her brains without just wanting to use her for them.
And now Usagi’s shown Ami that she’s at her most powerful when she wants to help and protect the people around her. She was a smart kid who was easy to exploit when she was alone and had no connection to anyone. A connection to other people and love and a desire to help them can focus her intellect and inner power and unlock her full potential, and that’s what Usagi has given her. Her love for Usagi and Usagi’s friendship and support helped her realize who she really is and what she wants to believe in and fight for. And so she transforms.