I think the most interesting thing from this site is that 7DMT and TSMM are the only RIDES (as opposed to shows and character greetings) that they even list because, according to the site, everything else is generally available same day.
This seems to refute concerns that someone cannot get any decent FPs 30 days out and cannot spontaneously change parks the day before, or even same day, because they won't get any decent FPs.
If I really wanted the spontaneity of being able to decide which park to go to in the morning, I would make FP reservations in advance based on a best guess of where I might want to go and then feel free to change if I felt like it. The most I would be giving up would be maybe a FP for one ride.