A 52-year-old man from Delhi presents with a 6-month history of a darkly pigmented lesion on his left shoulder. Clinical examination reveals an asymmetrical, irregularly bordered lesion measuring 8 mm with colour variegation (brown, black, and areas of regression). Dermoscopy shows atypical network pattern with irregular dots and globules. The lesion is excised with 5 mm margins and histopathology confirms cutaneous melanoma with Breslow thickness of 2.8 mm, Clark level IV, with no ulceration and mitotic rate of 3/mm². Sentinel lymph node biopsy is negative. What is the most appropriate next step in management?