Option 1: Can you incorporate it into the design of your built ins? I think it would look nice to highlight it, especially since it’s sentimental, if you don’t absolutely need the entire wall for storage. Pinterest has a lot of examples! Otherwise I think it will take away from the aesthetic of the built-ins, like your space and the things in it are mismatched.
Option 2: move the piano to another room. Think about all your available wall space, even a foyer or wide hallway. Just avoid keeping it in direct sunlight if you don’t want it to fade.
Option 3: place in front of the built-ins, but with the keys/bench toward the them, not away. You can buy a felt backing to protect the soundboard and make it look more finished, or leave it as is. I saw this done where someone had painted the piano with chalk paint, including the back frame. They arranged the room so a couch was perpendicular to the back of the piano and it looked good. But, I think the exposed back would only work with some home decor styles.