Agree more time to Sedona because March weather up on the Colorado Plateau can be sketchy.
But what I would suggest is thinking in terms of "trip segments" and how to leverage Flagstaff vs. Sedona during your March stay. March weather will set your real trip parameters so be ready to flow with it. This year we had some many cloudy, wet, windy March days here in Sedona and up on the Colorado Plateau including more snow up there.
If you want to hedge your bet, starting in Flagstaff not a bad idea since GC is your # 1 destination. Having a few days offers opportunity & time to altitude acclimate & achieve your GC goal sans worry.
Think Petrified Forest
as an alternative for a so-so weather day to go for a ride to escape cabin fever waiting to do GC. Likewise, an alternative cabin fever drive could be a visit to Ash Fork
west of Flagstaff on the I-40. AshFork is on part of Route 66
& has a nice little museum
. Nothing fancy, but interesting. Once again, simply a way to rescue yourself from bad weather and still hit the road & see the AZ landscape.
remains your 2nd main Flagstaff goal weather allowing, assume a 5 hour drive in case there's some road issues. As you say, a long arduous days drive to Zion, . We do something similar years ago driving from flagstaff to North Rim of GC thinking we could make the RT that day.
We learn otherwise of course as we lingered in the beauty. We luck out finding a vacant room 20 miles north of the rim. So keep eyes open for any motels along the 89 on the drive up just in case. We're glad we spent a few $ to linger and avoid driving red eye.
Sedona is 40+ miles south of Flagstaff via I-17.
If the weather gods are really unkind during your visit & mar your GC dreams, keep in mind you could drive up to Flagstaff night before & stay in a hotel to get an early start to GC in good weather. Yes a few extra $, but worth it. Then again I speak of how we traveled and explored this region before we move from NJ to Sedona 6+ years ago.
If you get out here and want a short but beautiful hike that won't put you on a ventilator give a holler as we have a nice doable trail here in Village of Oak Creek where we are riding out our life's rodeo.
enjoy playing your trip