When the game is running, farmers will prefer to tend the farm rather than forage for berries. That's just how they are. You can micromanage them, or leave the game off until the next crops appear.
15 villagers are not going to be well fed with only level 2 of Farming technology. I rarely allow my population to exceed a dozen or so villagers until my villagers have learned to fish.
The reason that the crops look random to you is that their cycle is determined from the time the crops
appear, not from the time that they are depleted. Since you have so many villagers, and your farmers are gathering all of the food quickly, the farm will be empty for a longer period of time than if they harvested more slowly. There are a fixed number of crops to be harvested from each crop cycle, and picking them all really fast won't speed up the appearance of new crops.
