> slavery or something similar (serfdom etc.)

Serfdom is very different from slavery. Even slavery is not always as bad as Roman slavery.

Roman slaves could be legally killed, tortured and raped (even children). Serfs might not have fair access to the law but at least in theory they had recourse and society recognised mistreating them was immoral.

Serfs could (meaningfully) marry. They were tied to the land so could not be separated from their families and sold elsewhere.