Best route from Portland to Garlech

Hey, I'm not sure if this is the best place to ask this question, so if not, I apologize in advance. I'm hauling a heavy load from Portland, Or to Garlach, Nv and am a little unsure as to the best route to take. I'm likely to hit below 25 mph going up a steep hill, so I'd like to avoid hills as much as possible.

Should I take I5 to Eugene, then cut east to Kalamath Falls, then through Alturas? This seems to be the most direct route, but I could stay on I5 all the way to Ashland then cut east to Kalamath Falls, which isn't that much further and keeps me on the freeway longer. Or maybe I should I go all the way down to Sacramento, then over to Reno then north? Google thinks this would add about 2 hours to the trip, but it might save time as it's mostly freeway driving.

Thanks for any advice.