I spent a lot of time in Alaska but always got there by plane or ship. I like the idea of cutting across Canada to get to Alaska. If it were me I would go that way and get to Alaska first. In the North reservations are not really warranted because you can always boondock everywhere.
I cannot imagine planning out a long trip ahead of time with reservations. The beauty of a long trip is that there are no time constraints. One of my favorite guidelines is "find a place you like, stay for around 3 days."
You will have plenty of time to visit sites in the Lower 48 when you return. You have the rest of your life to revisit them. It is the time in Alaska and Canada that you may not get to again.