Steve -
You may wish to start thread about Mount Rainier N.P.
I've biked most of both routes you've posted.
I rode Mount Rainier NP in 2019.
Mount Rainier NP is one of the least bike-friendly national parks in the U.S.
I don't get it - because it's in the progressive Pacific Northwest.
There are zero hiker/biker campsites - making summer camping very tough.
Plus traffic on Paradise/Stevens Canyon Road is heavy including busses & RVs.
Double plus, there are no shoulders and some mondo climbs.
2.5 million visitors in 2021 - probably 3 million in 2022.
And it's frequently done as a day loop out of Seattle.
(BTW - Paradise Valley parking looks like a mall and the nearby trails are packed.)
Two park options -
a) If you want to do some hiking in the park consider Sunrise Park, just north of Cayuse Pass.
You would need to camp at White River Campground and, almost certainly, need advance reservations.
Then it is a huge climb up - but the hiking and views are magnificent, esp. in the morning.
b) A great edge of park option is the Naches Peak Loop at Tipsoo Lake.
Camp as far up the American River as possible - Lodgepole Campground.
Then start up Chinook Pass super early - the climb will warm you up.
Why? Because the morning sun on Mt. Rainier is amazing - do the hike clockwise.
Either a or b would mean missing the main park road, the traffic & the climbing.
Plus it puts you ending in Astoria with easier connections.
US 12 west of Packwood has good shoulders, but there are nice back roads, too.
Hwy 4 west of Longview has moderate traffic, huge views of the Columbia River.
Skamokawa has a county campground that is nothing short of breathtaking.
I would stay on the norhter side of the Columbia to Ilwaco and the coast.
For a finale, Cape Disappointment is way better than either Westport or Astoria.
All three have state parks with hiker/biker campsites.
Westport - Twin Harbors SP - quaint little town, paved bike trail along the dunes, flat.
Astoria - Fort Stevens SP - more services, old shipwreck on the beach, flat.
Cape Disappointment SP - on the Washington side across from Astoria, rugged coastal cliffs.
Connections from Westport to Aberdeen to Seattle are limited and complex.
Connections from Astoria - 5 thru busses per day.
https://ridewithgps.com/routes/38690851