Best of the Western US (and Canada)
Welcome aboard the RoadTrip America forum.
First off, you're looking at around 3750 miles in 21 days, so that is not a big strain. It averages to less than 200 miles a day. That means you'll actually have some time to enjoy the places you're going to be spending so much time getting to. So as long as you and your companion don't mind driving a moderate amount most days, this could be a very enjoyable, and quite scenic trip. The one thing that's going to be missing is any in depth understanding of any of the great parks you'll be seeing. But as long as you understand that this is a smorgasbord kind of trip - go for it. Just be sure to get a National Parks Pass either before you leave or at the first park entrance that you come to. It will end up saving you quite a bit of money. Also be sure to stop at the visitor centers of each of the parks for advice on which hikes will best meet your desires. These people are a wonderful resource and will have up to the minute information on the conditions of their parks, particularly important during fire season. In any event, you will have a tough time finding non-great hikes or non-scenic routes on this trek. You might also want to have a look at posts by moderator Judy on the Pacific Northwest, such as this one.
AZBuck