A 64-year-old man with reduced ejection fraction heart failure (LVEF 30%) is stable on Furosemide. He is breathless on minimal exertion and has ankle swelling. Blood pressure is 120/70 mmHg, eGFR 70 mL/min/1.73 m², potassium 4.5 mmol/L. What is the most appropriate next step in optimising his chronic therapy?