The OP's post has one mistaken asssumption and one questionable assumption.
The mistaken one is that touring bikes are somehow built for "long" but not "steep" rides. Touring bikes have low gears, otherwise they are not touring bikes.
The questionable one is that the OP's knees were bugging him on a 25 lb bike and that the problem will go away with a lighter one. Well maybe not. Climbing 7,000 to 9,000 ft on a regular basis is tough. I'd get the knee checked out and I'd think about how to train for that kind of riding.
That said, if you want a lighter bike, maybe you don't want a typical touring bike made of steel. There are lighter bikes out there and by going ultralight with frame bags, you can take most any bike you want "touring."