Victoria (and the ferry ride over) are well worth the trip.
When I planned my first trip up to BC, I searched an annual average monthly weather report.....Aug is their best riding month (least amount of rainfall and best temps) however, you're still subject to rain even in Aug.
Make sure you have your passports and medications intheir original prescription bottle(prescriptions for each).
You've got a great route layed out, I'd just suggest adding Victoria. You could catch a ferry in Seattle and go over to Bainbridge Island, ride north across Hood Canal Bridge to Port Angeles (a beautiful ride). Then catch the ferry over to Victoria.
From Victoria (be sure to see the Butchart Gardens and Craigdarroch Castle in Victoria) head north to Departure Bay where you'll catch the ferry back over to Horseshoe Bay (just north of Vancouver). There you'll want to stop at the Capilano Suspension Bridge.
Just a few more suggestions for you to consider for your route. It's all an incredable ride.
I have taken a rental car across the boarder. Don't think a motorcycle would be any different.
You might want to reconsider the points that Bungy makes. The same applies to riding across North Dakota and much of Wyoming. It can get pretty boreing out there (and very hot to)