A 68-year-old man with heart failure with reduced ejection fraction (LVEF 28%) is on ramipril, bisoprolol and spironolactone at guideline-directed doses. He remains NYHA class II and euvolaemic. His eGFR is 66 mL/min/1.73 m² and potassium 4.5 mmol/L. What is the next evidence-based pharmacologic addition?