A custom button starts before the circular crop. It starts with what the image needs to keep: a face, a word, a gesture, or the quiet space around a mark.
The next choices are about balance. A detail that looks clear on a large screen may need more breathing room on a small object. A photograph may work best when the subject is allowed to stay whole instead of being pushed into the center.
The useful question is not “How do we fill the shape?” It is “What should someone notice first when they hold it?”