Would you agree that Iran was once Sumer?
No. Because it’s not the case. Sumerians were not Iranian. The Sumerians were in Mesopotamia (Iraq) and at an way earlier date. The Persians came much later and at an totally different location (Iranian plateau). The Persians entered Mesopotamia first after defeating the Babylonians. There were the Elamites (related to Iranians) who existed next to Sumer and interacted with them. But they weren’t the Persians we’re talking about and Sumer never controlled what is Iran or Persia, except some city states at the Gulf.
The whole region (which was once Sumerian, Assyrian, Babylonian etc.) became part of the Persian empire in the 6th-5th century b.C.
(Ah... I posted my last comment without finishing it.)
If one look at the “Cylinder of Cyrus” where the rights of the Persian Empire were declared, it clearly abolishes slavery. This becomes more than evident with the liberation of the Israelites from Babylonian Slavery by the Persians, as evidenced by the Bible. Scholastic works only attest that defeated enemy armies were used for labour in captivity.