advise needed

Replace bamboo with sugarcane. Connect all pistons with redstone. Add observer on top that looks at daylight sensor and connect it with rest of the redstone. That will activate all pistons few times during the day and you don't need observer for every sugarcane. You can add one more observer with daylight sensor set to night mode so farm will activate during the night too. This won't improve the rates (if we don't count replacing bamboo with sugarcane) but it's much cheaper than having 1 observer for each sugarcane like others recommended, especially for bigger farms.