You won't do *everything* unless you stay for multiple weeks. Our trip is booked for Sept. 2020 as well, and we'll have 6 full days plus 2 travel days. Our arrival day should have us at a park by 1pm, while we leave 4pm on our last day.
Our plan is to devote the last day (6 hours) to Disney Springs. That will in no way come close to being enough time to do all Disney Springs has to offer, and that's not even technically a park. Even if we gave Disney Springs an entire day we couldn't do all that's there--and by that I just mean all of the activities and shops. Clearly we wouldn't be able to try every eatery in a day.
We plan to devote at least one full day to each of the 4 parks, plus a half day at each water park, leaving us 2 half days plus our arrival day as extra time to devote to returning to some parks a second time. We won't see or do everything in the 4 major parks by any means.
We may or may not do all the water parks have to offer (main things, not separately paid for extra events) depending on how determined we are, what's open, and how the crowds are. (Which brings up another point---there will be plenty of things closed in Sept. 2020, especially in Epcot. So it won't be possible to do those things no matter how long you stay.)
That means we also have no time left for the golf courses or resort hopping or heck, even special activities at our home resort for our stay! And there is so much to do at all of the different resorts!
So, for a first visit, if time and budget allow, I'd try for at least one FULL day per major park, just to get a feel for them, an overview if you will. I would not go in with any "do or die" rides or activities in mind. Just go and enjoy what I could. Then if you can stay longer, you can devote your extra day(s) to revisiting the parks you still have the most left to try, or the park(s) you like best. If you want to do more than the 4 major parks, then I would try for a trip beyond the 4 full days plus a few extra...maybe 8-10 days? But even then you won't do everything. You'll just get to try to get a taste of more of what the entire property has to offer.
My personal opinion is to try not to get your self bogged down with an insane amount of planning for a first trip. Try to give yourself time to be able to take in the parks as a whole, not just a mad dash from one item on your itinerary to another. (Although I know other folks would feel a trip without an action-packed battle plan is a waste, which is why I said it was my personal opinion.
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