Google Maps 1st choice crosses the Rockies via the Crowsnest. That's about as low and gentle a pass as you can find. Then US95, US395 to I84 to Portland. No passes that way, though you might hit wind, dust and blowing snow across central Washington. And strong winds through the Columbia Gorge if there's an Alberta cold air mass behind you. Then I5 to Grants Pass, and US199 to Crescent City. 199 winds a bit as it follows the Smith River in California, but takes a tunnel under the highest point near the state border.
An alternative would be take OR42 from Roseburg to the coast. A little longer, but lower.
There isn't anything more direct, even if you were willing to cross some passes.
