Every form of slavery is slightly different. That does not mean they are not slavery. Someone who is worked to death doesn't care about the specific name. If you do not choose to be a servant, you are a slave, and many of the people who have been called indentured servants were actually indentured slaves. What is the difference between an Irish prisoner of war who is sold and an African prisoner of war who is sold?

And you are simply wrong when you claim indentured servants always had more agency than slaves. You need to read more about life before 1680.