The difficulty with your position is that you have no other source of "truth" than your Scripture, a number of different interpretations of what that Scripture means, and no way to determine which is correct other than through the Scripture that you disagree on.
This is the "true Scotsman" argument.
You say "Christians think this."
Other Christians say "we believe otherwise."
You say "well you are not true Christians then" (or are, at least, mistaken Christians.)
You are right that tradition alone is not proof of truth, but the historic consistency of Christian teaching, the life and resurrection of Jesus, and the ongoing transformative power of his message provide compelling reasons to take the Bible seriously as more than a flawed human text.
I think you are seriously underplaying the resurrection of Jesus here.
If Jesus died and came back to life and was both god and also the son of god, then you have a very compelling reason for us to rethink everything we know. If that is true, then everything we know about physics, biology, cosmology, existence is wrong.
But the only evidence for that is the Jesus story is written in what very much appears to be no more than a flawed human text.
And a text that claims to be the ultimate book of human morality which doesn't reject slavery, but in your words "lays the foundation for its rejection." Flawed, indeed.
Early Christians could not abolish slavery in the Roman Empire overnight.
Irrelevant. I am not commenting on the actions of Christians. I am commenting on the lack of moral certainty over slavery in the Bible.