There are more civilizations in human history that had slavery than the ones who did not. So slaves can not be the differentiating factor in what the Romans achieved.
Agreed. There's nothing inherent about slaves that is more efficient than paid laborers. It's just cost and time. It can look incredible that the Romans built these huge structures, but the timescales were measured in decades. Same with cathedrals in the middle ages. If you've got 200 years to build something you can really do a great job.
