SY -
I have a great deal of experience riding in South Dakota and Wyoming.
I first biked across South Dakota in 1989 and have lived in Wyoming since 1990.inest riding
I've cycled across the Great Plains many times - from Canada down to south Texas.
Without a doubt, the Nebraska Sandhills offer the best route and riding experience.
Because they have poor soils, they were never plowed and retain natural grasslands.
The roads tend to curve and sway around and over the hills -
rather than go due east-west for 300 miles through cornfield after cornfield.
The Black Hills are a nice interlude, but have heavy tourist and second home development.
The Mickelson Trail - although unpaved - is a quiet interlude. Rochford is a hoot.
Northern Wyoming offers Devils Tower, the Bighorns, and Yellowstone - some big climbs.
But you should have experienced legs by then.
Have you been to Glacier National Park - ridden Going to the Sun??
It would mean missing Missoula or taking a big loop back down.
US 89 - and back roads - makes a fine run up from Yellowstone.
Then you can connect with the Northern Tier or zig down to Missoula.
Just some ideas. - - Jama
PS -
Be aware in your planning that there is much resistance to the Covid vaccine in the region.
If you have a completed vaccine sequence, you should be O.K.
But I fear that Covid will persist here for some time into 2021.
Pic - Going to the Sun Road