I wouldn't describe WA14 as hilly. Yes, there is a climb at the west end around Cape Horn. The views on that climb are worth it. Otherwise it basically stays at river level, just like I84. It's no worse than any of the drives to the coast.
Still, if all you want is an easy way through the Gorge then stick with the freeway. The waterfalls are on the Oregon side, but you have to get off the freeway to see much of those. WA14 gives a wider view of those Oregon mountains, and has some stops of its own (e.g. Beacon Rock).
If you want more scenery, I could easily weave you a route through Goldendale, Klickitat, or even Trout Lake.
Or stay on the east side of the Cascades to Bend and cross on 20/22, avoiding the Portland area entirely.