If you have toured Firenze-Nordkapp and across Australia, then you don't have to stick with ACA maps.
They are good and provide reassurance for novice cyclists - but you are experienced.
Given the number of days you have and the time of year - you should pick what works best for you.
Starting in Florence on the Oregon coast would be nice - but the coast is not very interesting there.
The amazing part of the Oregon coast is just to the north - between Newport and Florence.
(It's easier making connections from Portland to Newport than to Florence, too.)
Oregon has special hiker/biker campsites for $5 and is very bike friendly.
From the coast you can follow the TransAm almost all the way across Oregon.
That would be a nice way to bump into other cyclists and get started comfortably.
The cut-off towards Idaho is in eastern Oregon - staying on US 26 to Ontario.
Here is a map of traffic volume for Oregon -
As you can see, there is very little traffic on the section of US 26.
Going through central Idaho is beautiful - Sawtooth Mountains - with snowy peaks and hot springs.
Also, there's lots of free camping on public lands - (NOT in National Parks).
I'll go over details of the Idaho section to Yellowstone in another post.
Photo - Redfish Lake in the Sawtooth Mountains