A 68-year-old woman from Mumbai presents with a 2.5 cm dark nodule on her right shin that has been present for 8 years without significant change. On examination, the lesion is firm, dark brown to black, and dome-shaped with a well-demarcated border. Dermoscopy shows a homogeneous pattern with globules. Excisional biopsy reveals nodular melanoma with Breslow thickness of 3.2 mm, ulceration present, and 8 mitoses per mm². Sentinel lymph node biopsy shows micrometastases in one right inguinal node. What is the most appropriate next step in management?