It sounds like you did a great job amending the soil. And, hydrangeas flower best with some sun. The best way to tell if you are overwatering is to feel the soil. If it is mushy and kind of smelly, then it probably is getting overwatered. If so, back off on the watering. The soil should actually dry out between waterings. A good soaking every 2-3 days should be enough. However, if the bottom leaves on the plant are yellowing, then it is a sign of not enough water. This can happen when the temperatures spike to the 90’s and 100’s. When this happens, extra water is much appreciated.
Thanks for writing!