We always go last week in june / first two weeks of july.
The crowds really aren't that bad, although, you may think so coming from a quieter time of the year.
As has been said, if you do things properly, knowing you won't get everything done, getting to parks for opening and making most of EMH and fastpass then you'll be fine. We normally go in first thing - no point going in later as will be busier - and come back to riverside around half 10 time - pool and then room in the middle of the day. Normally go out to parks tea time for evening.
There is no point going for rides in evening, as no fastpasses and will be high standby lines. That teatime - fireworks time is the time for shopping and shows - philharmagic etc. Not sure where you go to watch Illuminations from, but if it is the Rose and Crown then you really got to be there for 7 to get seats.
It will be hot and humid, but we've never found it too much - you will of course if walking around the parks in the middle of the day. Expect the thunderstorms in the afternoon, which usually do clear through quite quickly, but sometimes will linger into the evening.
The only parks to reach capacity in july are blizzard beach and typhoon lagoon along with MK on the 4th. However, that isnt a problem as for the last 7 years they've shown the MK4th show on the 3rd and we go then and to EPCOT on the 4th. This year there is a whole week of July 4th events and fireworks on the cards.