The HCRH is at best moderately busy on a weekend, but once you get beyond Corbett, it's all tourist traffic, meaning everyone is doing the same thing you'd be doing: stopping at every waterfall and viewpoint. And it's highly scenic. The roads in the Coast Range would be quieter, and depending how you approach it, more challenging.
If you went out to Scaponia from Hillsboro MAX (Hatfield), you'd be looking at 40 miles of riding. From Cleveland Ave MAX in Gresham to Ainsworth State Park, 25 miles. Sunset on Saturday October 4 is 6:45 pm.
Just looked at your Day One itinerary. That route could be very rough and might require GPS to figure out. I've never gone that way, but someone like Ed may have knowledge. In any case, that's probably a pretty advanced route option, something not to be taken lightly, but could be very rewarding. If you want to work for it!