A 48-year-old Indian woman with a 10-year history of type 2 diabetes mellitus presents with progressive velvety darkening of her neck, axillae, and inframammary folds over the past 2 years. She denies weight loss and reports stable weight around 78 kg (BMI 32 kg/m²). Her HbA1c is 7.8%, and fasting insulin is 24 mIU/mL (normal: 2–12 mIU/mL). Skin biopsy shows acanthosis with papillomatosis and mild hyperkeratosis. Which of the following is the MOST appropriate next step in management?