A 38-year-old woman presents to the gynaecology clinic with complaints of heavy menstrual bleeding for the past 8 months and progressive abdominal distension. She reports passage of clots and dysmenorrhoea. On examination, the uterus is enlarged, firm, and irregular with a mass palpable in the lower abdomen. Haemoglobin is 8.2 g/dL. Transvaginal ultrasound shows multiple hypoechoic lesions within the myometrium with heterogeneous echotexture, some with cystic degeneration. What is the most appropriate first-line medical management for this patient?