To do this subject justice would require a depth of study that nobody is motivated to fund. IMHO.
Agreed. There are, for example, several different compounds used for calcium. Some are more readily available than others. There are optimal ratios of different vitamins as some tend to work together, like Vitamin K2, D, and A.
Then there’s how they are manufactured. Different processes result in better quality absortion. There’s a great deal of other variables as well, such as one’s own genetic makeup. Probably too many, as you said, to find the funding to to it right.