You may stir new grains to get them started, but never mature ones while fermenting. If not quite ready when you are, just put kefir with grains in the fridge, it will be ready by morning usually. If it does go too far by accident, strain the curd -- it's like yogurt or farmer's soft cheese

I don't usually strain my product. Just spoon out the grains that float to the top and save em to use again. It saves a step. A few grains my be missed, but hey you got plenty and they are edible.

Make it thicker:

https://www.youtube.com/watch?v=INnWhv0Vsts