Won't comment on timing (though I think you have plenty), but would add a few suggested sights to see:

Cedar Breaks NM (between Zion and Bryce) - if it's still open by then.
Canyonlands NP - Between Arches and Capital Reef

There are some nice state parks in UT as well - off the top of my head near your destinations: Newspaper Rock, Dead Horse Point, Kodachrome Basin, Coral Pink Sand Dunes.

Whatever you do, have a blast - you're visiting one of the most beautiful areas of the country!

Sean