There really is no good way to bypass Portland on the L&C Route.
Portland is extremely bike friendly - but it is a big city, nevertheless.
The Lewis and Clark route bypasses most of Portland, even when in Portland. The only city neighborhoods that it goes through is St Johns and portions of North Portland, which has plenty of services, including bike shops and groceries. But they are mostly residential "streetcar suburb" neighborhoods and not really "big city" like. The rest of the way through the metro area is along Marine Drive, which is nice because it's pretty straight and flat and much of it is separated bike path. But you'll see very little of actual Portland unless you detour from the route. If you don't detour, you may wonder why everyone makes such a big deal about Portland.
Finally, I rode Hwy 14 on my first X-USA trip in 1987. It has way more traffic now.
I've ridden Hwy 14 a couple of time since and have driven it, too.
Definitely agree on the "more traffic". Quite a bit of semis, not too much shoulder either, at least from the Camas to Bingen section. East of there, not as bad. And if you ride 14 the whole way through the Columbia River Gorge, you miss most of the scenic attractions.