We've done late August a couple of times in the past and are going Aug 24-30 this year before taking a
RCCL cruise. The things to note are:
Crowds will be dramatically lower and very managable from just a couple of weeks earlier, as most areas of the country (outside of the northest) are already back in school.
Yes, it will be hot. Just tour smartly...alternate indoor and outdoor attractions, drink lots of water, bring a misting fan...and you'll be fine. Don't go by just last summer, as it was unusually hot - even by Florida standards. Can get showers/storms (usually in the afternoon), but it shouldn't ruin your day.
Parks do start closing earlier. No midnight closings at MK, for example. 10pm is probably the latest you'd see. AK closes by 5, DHS by 8.
ADRs book up quickly (and make it seem like it wil be busier than it really is) because a lot of people make ADRs simply on the hope/speculation that they will eventually offer free dining for that time frame (they used to regularly in the past, but I don't think they did last year until after Labor Day - someone correct me if that's wrong). A lot of ADRs will likely open up as we get closer - especially if there's no free dining.