A 50-year-old man with a 3-year history of Menière disease has been on sodium restriction and hydrochlorothiazide 25 mg daily for 18 months. Despite compliance, he continues to experience 3–4 disabling vertigo attacks per month lasting 5–6 hours, with progressive low-frequency hearing loss and persistent tinnitus. Audiometry shows worsening unilateral SNHL. What is the most appropriate next step in management?