Harvesting is only predictable in a general sense, as it's going to depend on weather conditions and especially rain during the summer months. Best thing to is keep your plans. BTW, I would strongly suggest you look into subtracting a day from Yangshuo and adding it to another of the north villages of Sanjiang county such as Chengyang (the one with the famous bridge). You would proceed there after the Rice Terrace night. IMO, four nights in Yangshuo is overkill.