don't own a crockpot and literally make bone broth a few times per year. if i had to make it all the time, i would be laggard about it.
i use traditional stock methods and smoosh as many bones in my big pot as i can and cover with water. add chopped carrots, a few garlic cloves, peppercorns, dried thyme, bay leaves, maybe ginger, maybe a few small tomatoes. NO SALT. bring to a boil and reduce to a simmer.
for chicken and birds, i generally use oogly bits like heads, backs and feet. i cook them at least 12 hours.
for beef/lamb i use bones, either soup-bones or neck-bones, a foot if i can get it, tendons, oxtail, etc. -- whatever is cheap and gelatinous. i cook that at least 36 hours.
strain the solids out and reduce the liquid. portion and freeze. THEN you can salt and season more generously each time you want some. salting at the beginning can mean disaster.
As I ate the oysters with their strong taste of the sea and their faint metallic taste that the cold white wine washed away, leaving only the sea taste and the succulent texture, and as I drank their cold liquid from each shell and washed it down with the crisp taste of the wine, I lost the empty feeling and began to be happy and to make plans.