I 80 from Reno to Sacamento over Donnner Summer is about a 4000 elevation climb, two lane few passing lanes. Lots of big rig traffic. I5 at Shasta is a looonnggg climb but several passing lanes then a long downhill. Lots of big rig traffic.
You can take US395 to Susanville / Alturas. From there you have several choices. Alturas is fairly flat and desolate. If I recall, continue north gets you to Medford, OR. Take Susanville thru a series of smaller California highways to Weed just south of Mt. Shasta. Or stay on US 395 till you get to Yakima, WA. Take I90 west into SEA.
Enjoy Emerald City.