We were in that area 2 summers ago. Check the link below for my thoughts, if you're interested in the full story. In short, we spent 5 nights at "The Watchman" in Zion, in the middle of June. The heat was real, but with careful planning on what time of day you do which activities, it certainly didn't effect our trip. We wished for more time there, and would have had no problem filling 6-7 nights. But, we hiked daily. If you don't hike, 2-3 nights would be plenty.
Bryce is amazing, but worth only 2 nights. That gives enough time to have one night star gazing and one day hiking into the hoo-doos.
Arches, Capitol Reef and many, many others would be a shame to miss if in the area...but your time is short.