In early May expect snow at higher altitudes. Bryce for one. Minimal crowds tho. But the weather that early can get bad in a blink of the eye.
There is no way to find one central location to visit Zion, Bryce, Capitol Reef, Arches and Canyonlands. Maybe for Zion and Bryce but you'll spend a lot of time driving. This is especially true if you really want to see the parks and get some hiking in. I take it you are new to western parks and how big they are and how far apart they are. Canyonlands and Arches can be both done from the Moab area. For the west side of Canyonlands anyway.