I don't know your touring experience level or what you will be riding.
Gravel bikes are pretty good - I've toured 30+ years on a mountain bike.
But touring on gravel can be anywhere from pleasant to grueling.
Plus, weather can change an unpaved road from nice to awful in short order.
(I use a 33% reduction in miles on gravel - i.e. 30 miles paved = 20 miles unpaved.)
You are leaving from the west coast pretty damn early.
You may be dealing with rain in the lower elevations and snow in the higher.
It's already been a doozy of a winter in much of the West - cold & wet.
Your crossing of the Rockies is pretty low - still facilities may open late this year.
OR 22 is very busy - esp. on weekends. Good shoulders, except a couple of narrow bridges.
Hwy 99W sux. Why downtown Portland? If no need then go via the Wheatland ferry and Oregon City.
The Historic Columbia River Highway east of Portland bike route is gorgeous. Some kick-ass climbs.
The most spectacular section is from Hood River to the Dalles. SO WORTH IT.
But . . . The section between Cascade Locks and Hood River is still incomplete.
And you cannot ride a bike across the Hood River Bridge.
I've ridden WA 14 many times - in 1987 it wasn't too bad. Now???
Narrow, mostly no shoulders, fast & fairly heavy traffic.
East of The Dalles WA 14 isn't bad.
MP 273.3 to MP 287.5
RideWithGPS is not always your friend.
Mostly unpaved - may not be public. Not terribly scenic either.
Why not stay on WA 14 to Plymouth and north on Plymouth Rd?
Is Spokane a must?
There are much better ways to bike thru the Palouse Hills.
Spring is gorgeous - although you may be a bit early.
(I think your start date is definitely a week too soon, probably 2 weeks.)
Heading out of the Tri Cities, the Columbia Plateau Trail is brutal - chunky clinker rock.
Pasco-Kahlotus Road has very little traffic - desert scenery, then you can cut dwon to Winddust.
MP 365 to MP 385 is a WRF section - RWGPS imagination - rough dirt/gravel.
When nearby paved roads are almost traffic free and sweet riding.
I'll stop before getting to Idaho.
If Spokane is not a must, then you can cur over to Plummer and the Trail of the Coeur d'Alenes.
Jama
Pic - Palouse in early June