Well as you no doubt know, it's pretty much a personal choice so no right answer for all folks.
Having said that, IMHO that the best default decision for most people is to ride what you already own and know.
I'd stick with the GSA. You have already done plenty of long trips on your bike, are used to how it runs, sounds and feels, and have adjusted your riding to the weight. It's not a small bike by any means but as you point out the distances are huge and there are plenty of freeways and good paved roads to ride between the highlights where a big bike will shine.
The other thing about Canada and the US (how's that for a broad generalization) is that you can also get off the beated track and generally ride good quality gravel roads (forestry roads, etc.). Many of these roads see little or no traffic but are great riding in their own right and would pass for major roadways in some developing countries. Your bike will be fine for this type of off the beaten track riding.
Now if you are really seeking to go off road (trail) riding and think that would be a large percentage of your trip, then perhaps considering a very light bike would be worthwhile. If you expect that to only be a small percentage, you could probably make do with your big bike and some small amount of suffering. If you were really going to go hard core for a period of time and need to go ultra lite, I'd look at renting or borrowing for that brief period. If you come to B.C. to do this, I could lend you a 650 single.