Eric -
It sounds like you have been bitten by the touring bug -
for which there is no cure - except more touring.
It also sounds like you have a good deal of experience -
so, you make want to consider striking out on your own routes.
As for history, I am a historian and have always ridden with an eye towards history.
Thr Northern Tier bypasses one of North America's greatest historical sites - the Mandan villages.
That is why I suggest Hwy 200 in ND and the Knife River villages / Fort Mandan, near Washburn.
The Mandan culture was is the focal point of the Northern Plains until decimated by the smallpox epidemic of 1837.
Lewis & Clark stayed there - Karl Bodmer's aquatints of Northern Plains peoples are irreplacable.
Bodmer - "Mato-Tope" -
https://upload.wikimedia.org/wikipedia/commons/4/40/A_Mandan_chief_0046v.jpgI have also found reading regional literature on a ride to be a great addition.
Montana's great writer Ivan Doig - many choices - "This House of Sky" is superb.
Native American writer James Welch, "Winter in the Blood" - powerful.
Ole Rolvaag's "Giants in the Earth" - a Dakota classic - Scandinavian settlement. (Tough read)
Kathleen Norris' "Dakota: A Spiritual Geography" is a gift
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What has been said above is what many riders experience on the Northern Tier - the Big Empty.
So you have to focus on other things - small towns, wildlife refuges, and as you saiis, the history.
If you don't have to get from Point A to Point B in a straight line, you have more options.
FWIW - US Highway 2 tends to be the straight line.
If you are going west-to-east, I'm guessing you will be starting in June.
I wouldn't start in May - I've been caught in heavy snow in the Cascades starting June 1.
This year, I started June 1 and was cold & wet all the way to the Idaho border.
If you haven't done the western segment of the TransAm, it really is beaituful.
But early/mid June is also when the Palouse is a carpet of green and empty back roads.
And Palouse Falls is roaring with snowmelt in June - almost dry by August.
If you haven't done the Lochsa River route in Idaho - US 12 over Lolo Pass - it is a "must"
Try to hit Jerry Johnson Hot Springs midweek to avoid the party crowds.
Unfortunately, it is a party spot for Missoula students on the weekends.
But the ride along the Lochsa River is simply gorgeous. A favorite of riders for 50 years.
The route from Missoula to Great Falls - Hwy 200 - is pleasant, but you miss most of the mountains.
For many people who don't bicycle, that would be a good thing.
But given how much prairie is ahead, you may want to get in a bit more mountain scenery.
Here is a suggested route east of Great Falls:
Kinda along the revised Northern Tier in Montana, but not in North Dakota.
(Actually, the central Montana section is closer to the Lewis & Clark ACA maps.)
It included sneaking in the back gate of Theodore Roosevelt National Park.
Which may be a no-no. ( I have NEVER done anything illegal in my entire life.)
BTW - Denton, MT was nearly destroyed by a wildfire last Dec. Some powerful history there.
https://ridewithgps.com/routes/41503269Pic - Palouse riding in mid-June, eastern Washington