Keep in mind that the OT laws were very innovative for their time. The eye-for-an-eye justice was actually an improvement over the previous ‘law’ which was blood feud that ran for generations in some cases.
What do you do with a conquered people? Realistically the choice is death or involuntary slavery. You can’t just release them into the territory you’ve just conquered. They don’t believe what you believe. They’re still functioning under blood feud and living, like you are at subsistence.
The kind thing in that instance and time would be involuntary servitude with the opportunity to convert. There is a solid tradition in the Bible of non-ethnic members rising up to positions of secular authority and also as prophets of God.
Joshua was instructed to exterminate all Canaanites. One group, the Gibeonites, tricked him into making a treaty.
So he didn’t kill them, just enslaved them and their descendants. They were much later slaughtered by King Saul.
I think it’s also relevant to point out that in the ancient world few people were really “free” in the sense we use the term. They were enmeshed in webs of obligation and duty, even if not in legal servitude. The actual condition of women in most societies differed little from what we would see as slavery.There were also intermediate conditions between slavery and freedom.
So slavery was not viewed, probably not even by the slaves, with the horror we view it. It was just a fact of life, one of the bad things that could happen to anybody that was unlucky.