A 58-year-old woman presents with progressive knee pain, morning stiffness lasting 2 hours, and symmetric swelling of the PIP and MCP joints bilaterally. Investigations reveal elevated ESR (68 mm/h), positive RF (1:320), and anti-CCP antibodies (120 U/mL). X-rays show periarticular osteopenia and early marginal erosions at the MCP joints. She is started on methotrexate 15 mg weekly. After 6 weeks, her CRP remains elevated at 18 mg/L and she develops new swelling in the wrist and ankle joints. Which is the most appropriate next step in management?