It is simply impossible for the mind of a 21st century white man (no matter how well educated), but immersed in 21st century values, morality, technology, health care, etc. to discern how a 17th, 18th or 19th century black slave assessed his quality of life in comparison with black slaves from other eras. Can't be done with any accuracy at all.
Consider further that were we to list and agree upon both the subjective and objective measures, or determinants of black slave quality of life, both the number and type of measures and the emphasis attached to each have changed across the last 50, 100, 150 or 200 years. That, in itself, renders the effort impossible
That's two variables that absolutely guarantee yours or my 21st century assessment to be, at best, barely an approximation, not anywhere near being a fact.
Certainly, being the objective, rationale person you are, you cannot disagree with that. Can you?