A 62-year-old man with ischemic cardiomyopathy has an LVEF of 30% and NYHA class II symptoms despite guideline-directed therapy with sacubitril/valsartan at target dose, metoprolol succinate at target dose, and dapagliflozin. His eGFR is 60 mL/min/1.73 m², serum potassium is 4.2 mEq/L, and he has no history of hyperkalemia. Which additional medication should be started to further reduce mortality and heart failure hospitalization?