Yes, you can overplan, or maybe over schedule yourself. Now, I'm one of the people who loves to plan their vacations and really loves to plan a WDW vacation. That said, I don't like to spend my entire vacation worried about rushing to the next thing and not enjoying where we are and what we are doing at that moment.
A few trips ago, I planned a lot of ADRs. We enjoyed them, but for some, it felt like the march of death to get us to go to them. After that trip, we reevaluated how we did WDW. We decided that the food wasn't that important to us. (I know a lot of people are probably falling over right now, but that is us.) This trip, I'm going without a single ADR. I may regret it, but we will also have a car, so if we want to go off property to eat a real meal, we can.
Instead, time for rides and non-food experiences are important to us. That is were I've focused my planning efforts.
And I totally agree - to have the best time, you need to be flexible. You may have the best, most awesome day planned, but if someone is cranky or sick, then that day is not going to happen as you planned. (And if you force it to, it will be even more miserable.) Or maybe you will get sidetracked with your DD's new love of XYZ, and spend a number of hours doing that. Go with it.