Hi Zach! Once upon a time there was a really wonderful group called CycleWild. Don’t bother to look it up; the domain is now owned by a domainer, and even its subsequent Facebook group is growing moss. A little taste of it remains in a few BikePortland articles like this, and this Google map with over 150 camping destinations within a day’s ride of Portland.
Stub Stewart State Park route comes close to your preferences if you take the MAX option to Hillsboro (end of the Blue line). From there to the hiker/biker campsites is about 20 miles, about half on very quiet back farm roads and the other half on the Banks/Vernonia trail, a paved rails-to-trails route. I don’t have a route sheet, and I don’t know where all the Cycle Wild routes are archived, but you can probably figure it out. From the Mark O. Hatfield MAX station, head north about 4 blocks on city streets in Hillsboro (pretty tame). Turn west (Garibaldi St?) into some older suburbs, then bear north again onto farm roads (NW 10th Ave?). Traffic is very light on those roads. Pick your way up to Banks. You’ll have to go on busy Hwy 47 but it has wide paved shoulders, and if you can get onto NW Wilkesboro, then you’ll only be on the highway for a couple hundred yards. Grab food at the grocery store on the south end of town, ride to the north end (half mile; easy city streets) and hop on the trail for a car-free second half to the park.