If you think you can just ignore the heat of the Southwest summers you are in for a very rude surprise and probably a badly ruined trip. Spring and fall are awesome but anywhere under 4,000’ from late May through mid September at the locations you listed is going to be seriously miserable more than not.
For the record I live in Tucson and travel extensively throughout the southwest. You need to be able to get to the higher elevations... June in Zion will be triple digit but up the road at Bryce Canyon fairly doable as just one example. We spend the summers at 7,000’ or higher whenever possible.
Add in the huge crowds most of the destinations you listed will have all summer(huge influx of European tourists plus the kids are all out of school) and the problem becomes clear. Even last April the ladies were waiting 45 minutes to use the ports potties at Arches Nation Park, I kid you not. Parking lots were all full and it took nearly an hour to just get into the park from the highway st 9 AM. Trust me summers are far worse. Sorry to be the bearer of bad news but better to read it here than to have your dream trip blow up.
I sincerely suggest you consider more research and hopefully can find a way to reschedule. Good luck.
