I have toured a number of times in Southern Utah. Your proposed loop is a good one, well tested. Only issues in March is that indeed it might be pretty cold in the higher elevations. Leaving Cedar City, heading up to Bryce, you climb to elevations around 9,000'. Cedar Breaks is around 10,000', Bryce 8500'. Might still be a lot of snow in those areas in March. I've done it in September -- pretty nippy at night but didn't have the lingering winter snow to deal with.