I've been making bone broth for years. I generally add the veggies and "whole seed" type spices on the second day to preserve the vitamins. Herbs go in only for the last hour or so to preserve their flavor.
Organic chicken backs & necks are a terrific way to get a collagen (protein) rich bone broth. I don't bother roasting them, just cover them with water, add some vinegar and let simmer. The broth isn't done till each vertebrae has fallen apart, the bones are "pock marked" and all the cartilage has vanished.
One editorial comment -- running a 1500 watt crock pot for two days is very expensive both in $$$ for electricity and to the environment! I make it in a heavy pot that I place over the pilot light on my commercial cooktop after it comes to a boil. The pilot light keeps the stock just below the boiling point without needing the stove turned on. I don't use the pilot light for its intended purpose. I light the stove with a match. But it is a terrific alternative to using a crock pot.