Welcome to RVF; glad to have you join us.
As for the best route from Yuma to "north of Seattle," you won't be able to totally avoid I-5. The I-5 leg from Portland to Seattle is really the only practical choice.
It kind of depends on how much time you want to spend on the trip. If you have time fo stop along the way, then I guess from Yuma, you could take 95 north to I-10, then up to Phoenix, across 60 to 93 and over to Las Vegas. Pick up I-15 in Las Vegas and go up to I-84. I-84 will take you along the Columbia River to Portland and you can take I-5 to Seattle and points north. I have traveled that route and it is decent, but will involve significant mountain driving. And, it won't be the quickest and/or easiest route.
Quite honestly, the easiest drive is to take I-10 to Phoenix, I-17 to Flagstaff and I-40 to I-5, then head north. That, however, puts you on I-5 for much of the trip and that's not where you want to be.
Not sure what else to suggest unless you want to take I-15 from Las Vegas all the way up through Utah and Idaho to I-90 in Montana and then west to Seattle. That will require negotiating a couple of mountain ranges in the process. Good road and not overly difficult passes on I-90. That's a nice trip and we do at least part of it (Montana to Washington) a couple times a year.
Good luck with what you decide.
TJ
