Definitely try to get an earlier start. If you can arrive before 11am you will have a better chance. Sunday will definitely be busy because everyone seems to travel on Sundays. Google maps shows about 3.5 hr, add some time at the Kennewick SC for coffee. Leave before 6am and you should get there by 10am. We used to pack the night before, throw the kids in around 4am and let them sleep until the first stop. Also, take 730 at Umatilla through Irrigon saves a good 5% over the interstate, so you can leave Kennewick with less charge.
Finally, since I hate big city traffic, I’ve taken the scenic forest route to Sandy (take NE Corbett Hill Rd about 5mi before Troutdale, then go through Aims) and it’s very pleasant, though slower and lots of curves. You come right into Sandy from the country, not realizing how much Portland suburbia has grown out to the Mountains. Then, take 211 through Estacada and Mollala, then 213 through Silverton to Salem. Did I tell you how much I hate Portland traffic?
Edit: Going through Portland to charge in Hillsboro is just crazy.
If you have to charge in Portland on this trip, Tigard makes more sense.
Your route sounds nice. Though I would probably go through Clackamas. I-205 is not going to be that bad on a weekend. I avoid I-5 unless I have to go somewhere on that side of town. I-5 has enough choke points that one accident and it's backed up for miles for hours, even on a weekend. There aren't a lot of sports this time of year, but when there is any kind of sporting event I-5 gets messy too.