RecruitingPhase 2Diagnostic testHERizon-Breast: A ctDNA-Guided Adaptive Study of Sequential Anti-HER2 Therapies and CNS Prophylaxis to Induce Long-Term RemissionThie purpose of this study is to find out whether a personalized treatment approach-using a series of ctDNA tests along with standard imaging scans to help decide when to step up (escalate) or decrease (de-escalate) sequential treatments (given one after another)-combined with local therapies (which treat cancer in a specific part of the body) and treatments that prevent cancer from spreading to the central nervous system (CNS; including the brain and spinal cord) can result in long-lasting remission and possibly cure some participants with HER2+ metastatic breast cancer.