A 58-year-old man with type 2 diabetes and hypertension has persistent albuminuria with an ACR of 35 mg/mmol despite good glycaemic control and ACE inhibitor therapy at target dose. His eGFR is 48 mL/min/1.73 m² and serum potassium is 4.9 mmol/L. Which additional therapy has the strongest evidence to further slow renal progression?