A 48-year-old woman with major depressive disorder has completed two adequate antidepressant trials over the past 14 months. She received a full-dose SSRI for 10 weeks followed by a full-dose SNRI for 12 weeks, both at maximum tolerated doses with confirmed adherence. Her PHQ-9 remains at 19 and she reports persistent anhedonia, fatigue, and difficulty concentrating. She has no psychotic features, no active substance use, and no significant medical comorbidities. The PMHNP is considering the next treatment step. Which of the following best describes the rationale for the recommended approach at this stage?