Yes I think those distances are crazy
and I enjoy roadtrips!
I would recommend cutting out Yellowstone or Northern California.
Leave in Portland and Seattle since you're headed that way anyways.
I would do SLC -Yellowstone - Boise - Hells Canyon - Portland -Seattle - Vancouver.
Or SLC - Lake Tahoe - Eureka/Crescent City - southern Oregon/Bend - Portland - Seattle - Vancouver.
Edit: September 7 is a major US holiday (Labor Day) so expect outdoors type destinations to be packed that weekend (5th-7th) Most retail businesses, restaurants, etc... will be open. Businesses like banks, post offices, etc... will likely be closed