I second the travel agent suggestion. They're free and they LOVE planning your trips. Tell them your must do's and they'll try to make it happen. At the very least, the planning bit becomes someone else's headache.
If not, I've booked my fair share of heavily planned trips and I've also booked a handful of last minute trips (within 30-60 days of arrival). I've loved each in their own ways. I didn't think I could be so spontaneous with Disney, as I was accustomed to booking my dining 180 days in advance and then booking the FastPasses 60 days in advance. I was just under 30 days from my expected arrival date for a last minute trip to get away for my birthday last year. I took whatever fastpasses were available, two dinner reservations (one at Paddlefish in Disney Springs and another at Ohana at the Poly), and told myself that we'd rope drop every morning, do what we felt like doing, and not overextend ourselves. It was a really short trip, a long weekend, and we wanted to try to relax a bit and not get caught up in the Disney type of 'rate race'. I was worried that I had sabotaged our trip, that we would struggle, that it would be overwhelming because there was such little planning. It ended up being one of our favorite trips. I had some idea of an itinerary prior to our arrival and while we were waiting in line for a ride our first day, we decided that we didn't feel like starting at Epcot the following morning, as I had originally planned and reserved FP's for. We wanted to rope drop Magic Kingdom instead, so I cancelled all of our FastPasses and rebooked whatever was available the following morning in Magic Kingdom (pretty sure I got Space Mountain, Splash, and Pirates...not bad for less than 24 hours in advance!).
As long as you are flexible with your dining, I think you should be fine. Your trips are typically 10-14 days long, I think you can get away with minimal planning as time is on your side. If it were a short, weekender, like mine...I'd get some ADR's for the places you're dying to eat at and just book the fastpasses whether you're committed to them or not. You can always change them, but I think they're a good Plan B for any trip. If you're going during a busy season, they're even more important to grab while the time is ripe (I had another last minute trip this past April the week immediately following Easter...those FastPasses were lifesavers). Just rope drop as often as you can. You'll get so much done so quickly and have the rest of your day to relax and wander while everyone else is scrambling.