I would also say that dark line you are seeing is a some sort of thermocline. Not the same type as forms in deep lakes in the summer, but where there is a quick drop or increase if lake has turned over in temps in a short depth change that causes the ducer beam to bounce back. It may be where the most oxygen rich water is and might be loaded with bait. I would suggest taking off the fish id on your finder and learn to read the arches. That fish ID is notorious for converting sounder feedback from leaves, weeds, debri, etc into fish symbols. If there is that much food down there that they are full, it might be why you can't get them to bite well.
