A 52-year-old man from Delhi presents with a 1.2 cm dark brown nodule on his left shoulder with irregular borders and colour variation (brown, black, and tan). Dermoscopy shows asymmetry, multiple colours, and irregular streaks. Excisional biopsy confirms superficial spreading melanoma with Breslow thickness of 1.8 mm, Clark level III, and no ulceration. Sentinel lymph node biopsy is negative. What is the most appropriate next step in management?