I am aware that many believed that slavery caused issues, such as job shortages, ostensibly because slaves took many of the low-level jobs, but I am not sure if any key thinkers of the age were actually opposed to slavery as a concept itself.
Not to dissuade any further answers but I would recommend u/captainhaddock answer to Is there any recorded abolitionist (anti-slavery) sentiment from ancient civilizations (ancient Egypt, Greece, Rome, China, etc...) ?
In addition to the other answer, I answered a similar question where I also discussed Gregory of Nyssa as well as Augustine of Hippo.
Here is a link to one of these conversations as well as a second one. Both links have other great answers in them as well.