A 72-year-old woman with chronic atrial fibrillation and a history of heart failure with reduced ejection fraction (HFrEF, EF 28%) presents to the cardiology clinic for rate control optimization. Her current heart rate at rest is 85 bpm, but she experiences palpitations with exertion. She has a contraindication to beta-blockers due to severe COPD. Digoxin was previously tried but caused nausea. Which antiarrhythmic agent would be most appropriate for rate control in this patient?