We have gone the last two July's and will be going this July as well. It will definitely more crowded than when you visited before Christmas. We have been at WDW for the Christmas/New Years season. It is by no means as crowded as that in July (except 4th of July time), but there will be lines for sure. We have used Fast Pass avoiding the ~ 2 hour waits at the popular attractions. It also gets pretty hot, so make sure you plan a pool/water park break each day. We had a group of 12 (with 8 WDW "virgins") go last July. I was the official trip planner and did a loose itinerary for each day. My BIL couldn't understand why I had pool/water park each day. Once we were there a few days he understood. My 6 yo niece learned to swim while at WDW and they had to drag her pruny body from the pool each day!
It has always been hotter back home than it is at WDW when we go. I'd rather be hot at WDW on vacation than hot at home at work! Our son is in 8th grade, so we can't take him out of school as easily anymore (although he did miss two days before Christmas break for a last minute week long getaway with me staying at Pop Century using that great $49 AP rate!!) so we adjust our vacation schedule to his school schedule.
As long as you don't have any unrealistic expectations about how much you will see/do each day, I think you'll be fine. Afternoon breaks are a good idea. Many of the parks are open late ( 9 or 10) each night so you can head back out in the evening after you've rested. Something that has ALWAYS worked for us is to avoid the EMH park of the day. Granted, our son is 14 so characters and "kiddie" rides (Dumbo, Peter Pan, etc) aren't a big deal to him . These parks always seem to be very crowded all day long. If you have young children and want to use EMH, I would say leave that park as soon as you have done your "must do" rides.
I'm sure you'll have a great time whenever you go!!
