Lots of people have done Hwy 21 - - it's on the Western Express.
http://www.crazyguyonabike.com/doc/p..._id=18807&v=Fg
The description says it all - esp. the possibility of water midway.
(You can always call the sheriff at Milford and ask if there is still spring water at the corral cabin in Wah Wah.)
You have a fairly high crossing over the Markagunt Plateau.
The lowest crossing is Hwy 20 - but it requires some interstate riding unless you do some dirt.
Hwy 153 is paved on the west side - dirt on the east side - fairly steep, but empty.
Crossing east of Cedar City is a tough climb to Cedar Breaks - paved, moderate traffic.
You should consider some combination of parks - Zion, Bryce, Cedar Breaks.
Zion gets awfully hot in midsummer - the others stay cold until late spring.
Cedar City has all services - Panguitch has most and is charming, albeit a bit touristy.
It's worth an out-and-back from US 89 to visit Bryce.
Or if you are really daring you can take the dirt road thru Grand Staircase-Escalante.
More importantly, I would urge you to consider riding to the North Rim of Grand Canyon.
It is really, really, really worth it - hiker/biker campsites right on the rim - services.
If so, you would cross the Colorado at Lee's Ferry not Page.
US 89A has way less traffic than US 89 - also US 89 is somewhat monotonous.
But, there is a mighty climb on US 89A to Jacob Lake - and then a big descent.
The chocolate cliffs west of Lee's Ferry are stunning.