I wouldn't necessarily agree that there are no headliners available for later in the day for your 4th, 5th, 6th, etc. FP. We went to MK on NYE - easily the most crowded day of the year I would think, and got great FPs for our 4-6th ones. That also was mid-afternoon on, say 4:30pm and later. I would say that is luck of the draw as to what is available later in the day. Don't completely count on headliners to be available, but also don't completely rule it out either. This is not to say that people have the experience of not getting headliners later in the day - don't want anyone to think I am negating their experience at all! Just relaying what luck we had.
I second the notion of taking whatever you can get and tweaking times later. For instance, we took Space Mountain just to have it, and a few days later I was able to grab 2 7DMT with just a bit of checking and such. We planned ours for 11:20-12:20, 12:30-1:30, and got 7DMT for 3:20-4:20 I believe. We had a nice easy morning (arrived at about 8am or so) and were able to stroll and jump on things before our FP started, and have a nice long lunch break. We ended up with 3 more FP throughout the night. I believe we got BTMRR again and HM (maybe not a headliner for some but the standby line was outrageous). It paid off to take what I got and tweak later - the standby for 7DMT was into the hundreds of minutes all day. Literally the highest number I have ever seen for standby besides EE when it first opened (a bit foggy but I want to say at least 150 or something - could be wrong).
We found it seemed as the day got later, people were canceling more and more really good things, I guess in anticipation of getting a spot for fireworks. I think mid-day FPs are nice, so you aren't rushing through the morning but aren't waiting to use them until nightfall.
I actually get to book my FP tonight, so this thread was super timely for me!