I've found Portland's bike map
to be rather helpful. 9th, 18th, 27th/29th, and 37th/38th are marked as low-traffic streets. While Williams has a gentle grade throughout, each of these avenues tend to have one block or so that is much steeper, with the tradeoff of being otherwise flat. Why not take Tillamook west to 18th, and then take that (and 17th) all the way up to Rosa Parks. Mind you, there is a hill around Skidmore of no more than a block or two, but at least the rest is relatively flat. Even walking up that one block would be quicker than riding all the way over to Williams. I hope this helps.