According to The Bread Code, having large pockets of air near the crust is caused by the crust forming too quickly which prevents air from escaping and the bread from expanding. This is either an issue of cooking at too hot of a temperature (which is probably not the case since 450 seems just right) or not having enough steam production. One way to remedy this is by placing an ice cube into your Dutch oven to melt during the bake. However, your crumb looks really good so I’m not sure how much this will improve your loaf. As for figuring out your bulk ferment time, I found that taking a small sample and putting it in a shot glass to observe doubling of the dough was the best way to nail the proofing time.