We have been to WDW in late August many times. My best advice is to decide how you will beat the heat. The heat zaps all of your energy and can easily turn minor disappointments into arguments etc.
We have toured with umbrellas, personal fans,
cooling towels, frozen water bottles, washcloths to wipe off our faces and necks, hats, etc.
Our plan was always up early for rope drop, parks until lunch in AC (at park or resort), then enjoy the resort and swim/nap/shower, enjoy a dinner reservation somewhere, and go to the parks until close. It doesn't work for every family, but even when the kids were late teens we all needed the break.
We have only ever stayed onsite and since we paid a premium for the resort we wanted to be sure to enjoy it. For us, it was as much a part of the vacation as the parks.
We also changed it up a bit and always had a resort night. We loved swimming at night and relaxing at the resort. Not sure where you are staying, but many resorts have nightly activities. Some have fireworks views.
If the break won't work for you, you could schedule a sit down meal in AC at a monorail resort on your MK day(s), a nice meal in World Showcase for EPCOT or at BW/YC/BC restaurants, maybe Rainforest at AK, and for HS in the park or you could still take the boat to BW/YC/BC. You could also take breaks by riding the monorail, the train at MK, longer shows like The American Adventure, Hall of Presidents, shows around World Showcase, etc.
I agree that splitting up at times might be something to consider. That might help everyone get to experience their must-do items.
The tip to bring more than one pair of shoes is great advice, too. With the daily afternoon storms, we needed 2 pairs!
We also gave our kids charging privileges on the MBs when they were in their teens, because it was easier for us when they wanted a frozen lemonade or a snack etc.
I hope you have an awesome trip!