I think your plan sounds like a great one. We were there in May last year (early-mid May, though) and we used TP. I found it to be an excellent resource. They always over estimated by just a bit which was actually ideal because then we had time to hop into a few shops, take restroom breaks, grab a mickey bar and still stay on plan.
And yes, they update real time while you're in the park. Get their Lines app. At any moment inside the park you'll be able to re-optimize your plan and they'll reset your steps based on current wait times. It's the best tool ever!
We also had fun providing them with our wait times in line. You push a button as you walk into line, then stop the timer as you board the ride. Helps them stay accurate and DH liked using that little feature himself to time our waits.
We're going again in May of this year and I'm using TP again, although I've set it to "very relaxed walking speed" since we're not using a stroller this year. I'm curious to know how accurate this adjustment will be. I'll try to remember to post a TP review after our trip.