A 28-year-old woman from Mumbai is diagnosed with smear-negative pulmonary TB based on clinical symptoms, CXR findings, and positive GeneXpert MTB/RIF (rapid molecular test). She is started on standard 4-drug anti-tuberculous therapy. After 2 months of treatment, repeat sputum smear microscopy remains negative, and she reports good adherence with no adverse effects. What is the most appropriate next step?