It's a tricky one, and I'm no lawyer. I agree, SA is the issue. However, 2C exempts all artwork from the category of "User Generated Content." I figure that takes even CC-BY-SA out the loop. The relevant chunks of the DMs Guild contract or whatever are:
2(c) “User Generated Content” shall be defined as the copyrightable elements included in your Work, such as original characters, scenes, locations and events. User Generated content shall not include the illustrations and cartographic artwork included in your work.
and
8(c) "Excluding the Owner’s IP and Program IP which we license to you and excluding the illustrations and cartographic artwork in your Work, you are the sole owner of all rights in your Work. You have the rights or license to use the illustrations and cartographic artwork in your Work."