Are these drying too fast?

It's totally normal for them to shrink suddenly on the first day, bro. They lose water from the cells and the pressure, that's why it looks like they shrink to half, but it's just pure water evaporating. Your parameters of 58% humidity and 64F are spot on for a slow dry of 10 to 14 days. As long as the fan isn't blowing directly on them and they stay sticky inside, everything is fine, don't freak out, they're not drying out too much. And an extra tip for your next grows is to add liquid silica from the vegetative stage; it's really going to help strengthen the cell walls of the stems and leaves, so the plant will handle heat stress, pests, and the weight of the buds better when they start to fatten up in flowering.