I agree with
@CaptainAmerica. Every day you add to your ticket is significantly less expensive than the first few. (The price difference between a 4-day adult ticket and a 6-day adult ticket, for example, is about $22!) Rather than going "feast or famine" and trying to cover parks in a single day, why not adopt a more balanced schedule each day? We like to arrive before rope drop and tour for about 4 hours, and thanks to the lowest crowds of the day and a good touring plan, we get more done in the first 2-3 hours than we could the whole rest of the day put together. We have a CS lunch, and as the crowds and heat are building to their most uncomfortable point, we're heading back to our resort for naps, and sometimes a swim, too. (Our kids are "too old" for naps and DH and I don't ever take naps at home, but at WDW, we find we all need them, and they leave us all cooled down and fully-recharged!) We don't head back to a park until at least 4pm, and we generally leave by 9pm, so the kids can get back to bed at a decent hour (except for the last night, when we all stay up late). Our evening touring is at a more relaxed pace: that's when we have the bulk of our FP+ scheduled, and we like to have a table service dinner during that time, too.
Structuring our days this way (with two 4-5 hour "touring sessions" and one 3-hour break) each day, we all stay well-rested and in good humor, and can cover 80-100% of each park with:
Magic Kingdom - 4 half-day sessions (doing every possible attraction, including Tom Sawyer Island)
Epcot - 2 or 3 half-day sessions (the number just depends on whether we want to see every film and pavilion in World Showcase or just some of them, and whether we want to do Ellen's Energy Adventure and the Innoventions buildings in Future World)
Hollywood Studios - 1 or 2 half-day sessions (depending on whether we want to see just a couple of live shows, or all of them)
Animal Kingdom - 1 or 2 half-day sessions (depending on whether we want to see all of the live shows and walk all of the trails, or just do a couple of each)