You've gotten some good replies here, particularly from Cicero in Post #6. Remember, too, that what we experienced in the United States, chattel slavery, was different from most other slavery that has been practiced throughout history. Slavery has been a widespread practice throughout most cultures and periods. The Bible did not forbid it, but did not condone it either. Like Cicero said, the Bible was concerned that the practice was just and slaves did have rights and recourse to some justice. Most of the book of Philemon in the NT is about a slave, Onesimus (sp?). Paul basically says to do well what you are expected to do in whatever circumstances you find yourself. However, if Onesimus's owner freed him, then that would be better. But it was not demanded.
American slavery evolved into something else. It was man owning man with few if any rights. It was an entirely different category of practice, IMHO. I do not believe that this type of slavery can be justified in any way by the Bible.
As you say, modern slavery tended to be different and worse than ancient slavery. Another difference was that modern slavery was racist. Blacks were viewed as different from other people, or as not really human. That was, strangely enough, a kind of Enlightenment scientific component. It was connected with the modern idea of Progress and with evolutionary theory, which argued that some "races" were inherently inferior.
In ancient times, it was the luck of the draw whether you were enslaved. Weak or conquered people ended up as slaves, but not because they belonged to a particular race.