A 67-year-old man with known benign prostatic hyperplasia (BPH) treated with tamsulosin presents with increasing lower urinary tract symptoms (LUTS), including poor stream, hesitancy, and nocturia. His IPSS score is 18 (moderate). Digital rectal examination reveals a moderately enlarged, smooth prostate. PSA is 2.5 ng/mL. Post-void residual volume on bladder scan is 80 mL. According to NICE guidelines, what is the most appropriate next step in pharmacological management?