"Perhaps start with a Shasta climb or a class in Spring."
For Shasta, you want a backpack that has a very tough bottom.
If you get a gigantic pack, then you will be too tempted to fill it up. Then the weight and bulk may defeat you on some important climb.
I took a 3000 cubic inch pack up Shasta one time, at least as far as high camp. Overnight, while in the tent, I had anchored the pack to the snow with my ice axe. Unfortunately, a wilderness ranger had been walking around, pulled up MY ice axe to move some ice, then replaced it into the surface, but the pack was no longer anchored down. The wind blew it away overnight. [That, in itself, is a testament to how lightweight it was.] All I had left was a flimsy daypack, so it had to serve me for the rest of the trip. I had to drag my expedition tent on a tether.