A 38-year-old multiparous woman with a history of two prior cesarean sections presents with severe dysmenorrhea and chronic pelvic pain for 2 years. Transvaginal ultrasound shows diffuse thickening of the junctional zone (>12 mm) with heterogeneous echogenicity and no discrete adenomyotic lesions. Serum CA-125 is mildly elevated at 42 U/mL. She has completed 8 months of combined oral contraceptive therapy without symptom improvement. What is the most appropriate next step in management?