A 52-year-old woman from Mumbai with primary open-angle glaucoma has been on topical latanoprost monotherapy for 8 weeks. Her baseline IOP was 26 mmHg (both eyes). Current IOP is 24 mmHg OD and 23 mmHg OS. She has no ocular or systemic contraindications to further therapy. Visual fields show early glaucomatous changes. What is the most appropriate next step?