Yes, it's possible to over-plan. And that's coming from someone who gets teased all the time by family and friends who know how much of a planner I am. So I get it, for sure.
Problem is, you can't plan for bad weather, a ride suddenly being shut down. Someone getting sick. Or having flexibility if you want to change a restaurant, don't feel like a ride/attraction at that time you planned, etc. Or it's 110, and you need a break. Lots of other variables too I haven't even mentioned.
So, the problem becomes "the plan" can become a dictator, and we are the minions who bust our butts to follow it to the letter. When we don't, we feel we let ourselves down. Don't get me wrong, for those that plan to only to to WDW once in their lifetime (for example) I would suggest over-planning.
However, since we've been there many times, it's just not worth the stress. Do we take park closing times and EMH in mind? Yes. Try to coordinate ADRs with what park we plan to be in? Sure. Take advantage of Fast Passes? Of course.
But beyond that, we know what rides/attractions we don't want to miss. We know we want to see SpectroMagic and Fantasmic. There's always plenty of time to mix in rides we know rarely have long lines, so they get fit in when they make sense. As a lot of you know, if you go often enough you sort of just know how to work your way through each park. I've found this is much more relaxing, allows time to just look around, people watch, etc.
Will we get to do everything we wanted? Maybe, but probably not. Much to my surprise I have learned not only to accept that, but I love it. It is vacation, not a check list. I have been on Peter Pan many times, if I miss it this year I can ride next year, or some other year. It will not ruin the Magic even a little.