A 56-year-old man with type 2 diabetes (diagnosed 4 years ago) is managed on metformin 1 g twice daily. His HbA1c is 8.5% (69 mmol/mol) despite good adherence and lifestyle measures. eGFR is 75 mL/min/1.73 m² and BMI 31 kg/m². He has no history of cardiovascular disease and blood pressure is 128/78 mmHg. According to current Australian diabetes guidance, what is the most appropriate next pharmacological step?