Zephirine Drouhin is one of my favorite climbing roses. The fragrance is terrific! I think it would be a mistake to prune out the newer stems and leave only the larger ones. Ideally, you'll have a kind of rotation, so that when you're pruning out canes, you're removing one or two of the oldest, fattest canes near the base of the plant. The younger canes are vigorous and just entering their prime blooming years, but as canes age, their bloom production may start to decline. When you remove an older cane, the plant will direct its energy into a newer, younger cane. Remove no more than one-third of the canes per year, and you'll end up with a more vigorous plant.