If you can, plan to stop at the Kiva Koffee House on highway 12. It is about 20 miles east of Bryce Canyon National Park in Utah. It is a "must see" if highway 12 is on your route. It is easy to miss because it is in the middle of nowhere, but that particular "nowhere" is spectacular. Here is the link:
I believe they are closed on Tuesdays, so plan accordingly. They also have two cottages where you can spend the night. I would put it on your route and then ride all of highway 12 because you won't regret it.
Monument Valley, east of the Grand Canyon, is also great if you have the time.
The "Going To The Sun Road" in Glacier National Park in Montana is the most majestic National Park I have been to yet. This road typically doesn't open until July because they get 70 feet of snow on that road and it takes a while to clear. It is also sometimes closed in September because of construction so call ahead and make sure it is open if you plan to ride it.
I got caught in a 3 day snow storm in Jackson, Wy in June the first time I tried to get there so check the weather on your destinations and adjust as needed.
Your loop looks great.
Here is a link to a ride report I wrote when I finally made it to the going to the sun road after two other failed attempts:
Another report on riding into Yellowstone National Park and riding Bear Tooth Pass. Here is the link:
Not trying to hijack your thread but hope the information will serve as a resource for you in planning.