A 32-year-old man from Delhi presents with a 4-month history of productive cough, fever, and night sweats. He was diagnosed with pulmonary tuberculosis 6 months ago and completed 2 months of standard DOTS (RIPE regimen). Despite adherence, sputum smear microscopy remains positive at month 4 of treatment. Chest X-ray shows cavitary lesions in the right upper lobe. Sputum culture and drug susceptibility testing (DST) reveal resistance to both isoniazid and rifampicin. What is the most appropriate next step in management?