There are several thing to consider here:

• Based on reading a lot of reviews from a lot of different sources one can learn that the Sigma Art series of prime lenses are very sharp when focused manually but tend to not do very well on Canon cameras in terms of autofocus. I probably wouldn't consider such a lens unless my intended usage was primarily as a manually focused lens.
• Autofocus performance basically has two components: accuracy and consistency. Accuracy describes how close to a target the center of a pattern of attempts is. Consistency describes how close to each other the attempts are. If a system is consistent by not accurate, it can often be corrected by moving the "point of aim" to compensate. If a system is inconsistent, there's not much that can be done. If the lens you have now is consistently the same amount off in the same direction and you trade it for another one, then next one may be more accurate but less consistent.

• The Sigma dock is only about $69 new. That's not very much in the grand scheme of camera/lens expenses. It wouldn't hurt that much to at least try and see how well it can deal with your problem.
• A lens that far "off" may be more than on the edge of manufacturing tolerances - it may be well on the other side of what is acceptable. If it left the factory meeting specs that it now does not meet, then obviously something has happened to the lens to put it in that condition.
• The lens is used and presumably has no warranty. If whatever caused the AF to be that far off starts manifesting itself in other ways...