I have posted often that Googlemaps is not always your friend.
I suspect that your primary route you posted includes sections of US 81 in Nebr and US 75 in Iowa. Yeeech!
Also, since most roads in the Great Plains follow section lines N-S or E-W, diagonals are rare and often busy.
One of the finest riding areas of the Plains is the Sandhills of Nebraska - and a S-N route can be stunning.
The Sandhills are a huge natural grassland that has never been plowed - remote, serene, amazing.
The little towns are 25 or more miles apart and they have only the barest necessities - but are great places.
Plus the Sandhills tend to be 5 degrees cooler with significantly lower humidity.
(All that irrigation for corn crops ends up in the air.)
Here is a suggested route from Julesburg, CO to Valentine, NE -
https://ridewithgps.com/routes/20223029Niobrara Bridge -
US 138 & US 30 are O.K. - light traffic and easy riding.
NE 61 north of Ogallala gets better, very little traffic, rollers.
NE 92 east of Arthur - even better - it's the rollers that make the scenery
Whitman Road - one-lane paved through magical country - the finest riding in the Great Plains
NE 2 - east of Whitman (only water available) - O.K., but after Whitman Road, hey.
NE 97 - another fine ride with almost no traffic, stock up in Mullan - pop.500
Great rail trail bridge over the Niobrara River just outside Valentine.
Remember, shade is rare in the Great Plains in mid-summer.
Start super early on hot days and quit by noon - maybe a few more hours in the evening.
Nebraska DOR County Maps website -
http://www.roads.nebraska.gov/travel/map-library/county/PS - You are not imagining 300 miles per day are you? I also don't think 30 mph is anywhere near realistic.
I plan 10 mph, average, with stops. Lightweight touring 12 mph. Killer pace 15 mph.
And if you get a headwind - subtract 2 mph off of all of those.