If time is not a problem, I'd recommend hitting BC5 after Kamloop.
It's a much longer way mind you, adds 3hrs easy to the trip on an already long drive. But you get to see both Jasper/Banff national park on the way south via hwy16 in their entirety (you drive through diagonally through both parks).
On the return, I'd suggest taking the long way home via the US unless hitting the Okanagan Valley was the idea, in which I'd take Vernon on hwy3.
The reason of coming back via US is to make the most out of the trip. SR20 is called "the most scenic mountain hwy in WA" for a reason. I did the drive once and IMO it totally worth the extra time. It's absolutely gorgeous in the summer (not accessible in the winter I believe).
Last but not least, it's going to be a crazy long drive. Maybe renting a car with unlimited mileage would make more sense.
