We do the week leading up to Labour Day, which for the last 3 years we've gone has been the end of August, this year it's actually early September. The 2 weeks leading up to Labour Day are when the crowds take a steady decline with a little peaking up again over Labour Day weekend before dropping off again after. So, crowd wise, your last week of August is pretty good. You should expect moderate crowds, very manageable. It is hot, no doubt. But personally, I love that. I know without a doubt that I am packing only shorts, I don't have to worry about carrying around a sweater for cooler mornings or evenings it's just hot all the time. There are so many opportunities in WDW to get breaks from the heat (restaurants, shops, indoor attractions), so it's not like you're out in the exposed sun/heat for 12 hours straight.
The only real negative for that time of year is you are in the height of hurricane season so you do need to keep an eye on hurricane trackers in the week or so leading up to your trip in case you need to make any adjustments to travel arrangements. We've done 3 years end August and one of those trips we were flying out when a hurricane was hitting the keys and flights were being cancelled while we waited in the airport. Fortunately, they only had to cancel southern Florida flights and we were able to get in as planned, but it was a good example of why staying on top of that would be important.