From sea level at I84, you will have to climb one way or other. Much of easter Oregon is 3-5000 ft, same for a lot of Nevada (Reno's about 4000).
US97 has an initial climb out of the Columbia gorge, but is relatively flat (ans straight) for the rest of the way south, with a highest point south of Bend (in Poderosa Pine country). OR31 is an interesting way of crossing from US97 to US395 (the 'Outback' scenic byway - scenic but not mountainous).
US395 from I84 has to climb over some low mountains before getting to US20 at Burns. I've crossed those mountains, but not on 395.
You can also get to Burns on US20 from Bend/US97. US95 is an easy drive from there. You could also pick up 95 from I84 near Ontario. US95 on the Oregon/Nevada border is about as a remote spot as you can get.
Other routes across the Oregon Cascades have been discussed and evaluated in other treads
