On that last part of your trip... from Twisp to Toppenish,WA.
I am NOT a fan of Blewett Pass. It's not scenic. It's a long gradual uphill drive that eats gas. It's not hard. It's just a very long uphill pull going from Wenatchee toward Ellensburg.
I would drive hwy 97 to Wenatchee... then go hwy 28 to hwy 281... connect into Interstate 90.... cross the Columbia River to Ellensburg and then Interstate 82 to Yakima and head down to Toppenish.
There are two things I want to point out. First of all, the beautiful overlook of the Columbia River on your RIGHT as you head west on I-90... Look for it after you leave hwy 281 behind. 2nd of all... at the same time... look on the ridge on the left and you'll see a metal sculpture of a herd of horses racing on the ridge. You'll want to stop to take it in. I think it's mile post 139 on I-90.
And if you want a beautiful campground...seek out Wanapum State Park on Huntszinger Rd (mile post 136 maybe) it will be just as you come off the I-90 bridge...on the WEST side of the Columbia River... SOUTH of Ginkgo Petrified Forest State Park.
It's a long uphill pull to get from the Columbia River to Ellensburg. No getting around going uphill. Does not matter whether it's Blewett Pass or coming this way... you got to do the uphill somewhere.
There's a really scenic (curvey) road from Ellensburg south to Yakima. It's Hwy
821. Lots of parking areas to pull off and get down to the river... but not all of those parking areas are paved. The drive is beautiful though and you'll be going downhill from Ellensburg to Yakima on it.
Kathy