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The most straightforward route from Detroit to Sedona is just I-94 to northern Indiana, I-80 to near Joliet IL, I-55 south to St. Louis, I-44 to Oklahoma City, and I-40 to Flagstaff AZ. From there, you should take the scenic route, AZ-89A down through Oak Creek Canyon to Sedona which, while it only two lanes and has some switchbacks as it descends into the canyon, is fairly level. This route is just under 2000 miles and can be made in 3½ days of solid but not exhausting driving. Such a pace would also have the advantage of having your overnights in metropolitan areas - St. Louis, Oklahoma City, and Albuquerque - with plenty of lodging options. Just be sure to stay on the west side of town so that you don't have to deal with morning rush hour traffic.
Now from Sedona to San Diego, you have some options. Yes, the shortest route has you taking I-17 to the Phoenix area, then I-10, AZ-85, and finally I-8 over the Laguna Mountains. Now, I agree that section over the mountains can be a bit daunting, but if you'd rather just put up with urban traffic you can do one of two things. Go back up to I-40 and take that to Barstow CA and then I-15 through L.A.'s eastern fringe to San Diego. Or you can stay on I-10 after Phoenix all the way to Beaumont CA and use CA-79 to cut down to I-15. The first option is about 550 miles, the second, just around 500, so both can be driven in a day and both are relatively flat.
AZBuck