A 35-year-old non-smoking woman presents with episodic wheeze and cough triggered by viral infections and exercise. Spirometry demonstrates an FEV1/FVC ratio of 0.68 with a 15% and 250 mL increase in FEV1 after salbutamol. She currently uses salbutamol as needed several times per week. According to the 2021 Canadian Thoracic Society asthma update, what is the preferred step-up controller strategy for very mild to mild asthma in adults?