Wouldn't Bryce and Zion be at least 3 hrs r/t off the route? Zion can't be fully appreciated unless you climb a peak in it (Angel's Landing being the lowest one). Add to that you need to take a shuttle bus into the park's main area (unless you're staying at the park lodge). Bryce is quite a bit further off the route? I think Cedar Breaks is less of a diversion (can see Cedar City from there).