The only planning we've ever done is to decide which park we're going to do on each day for the first week or so.
So if it's just Disney, the 'plan' might be:
Epcot
Magic Kingdom
Animal Kingdom
Hollywood Studios
Typhoon Lagoon
Blizzard Beach
Then we go back to each park to do anything we didn't get a chance to do the first go-around, or to repeat rides. The water parks might be done in the a.m, with a round of mini golf in the pm, or vice versa, depending on the weather. Or shopping, if anyone wants to go.
However- none of us ever wanted to do character dining or anything, so if you want to do those then you need to book well in advance.
If you have to go in school summer holidays, go for the last two weeks of August. Flights are often a little cheaper, and it isn't quite as humid in Florida. It will rain though, but that's ok- it rarely lasts long and is still fairly warm. (Having said that- the last two times we've been it has rained quite a lot on one or two days. Take several pairs of trainers to allow them to dry out.)
We've done onsite accommodation (Saratoga Springs) and villas offsite. Loved the dining plan when staying onsite, and the ease of transport, but prefer a villa because none of us want to share a room with each other!
We've done a mix, too, of car hire, shuttle busses and Uber/Lyft. Uber/Lyft actually worked very well- but we are a group of three adults and an older teen, so easier than it would be with young children and their gear.