The ancient Greek word for "Slave" is δοῦλος, pronounced "doulos". I'm struggling to see how that comes from "Slav". The sources I've looked up say the derivation is uncertain, but noone mentions "Slav" as a possible origin[1]

[1] https://en.wiktionary.org/wiki/%CE%B4%CE%BF%E1%BF%A6%CE%BB%C... and https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1... for example

Yes, but I believe it was Latin, not Greek (Constantinople was a capitol of Easter Roman Empire, and they used both Latin and Greek). Latin term Sclavi that meant Slavs started to be used for slave servants, because there was so many of them.