I'm not a doctor, but wouldn't slumping be an indication that your back muscles have atrophied too much from sitting down all day? No chair / back rest is good enough to prevent that from happening, and no chair will be comfortable enough if the problem is a weak back. In fact, the more comfy and supportive the chair, the more your back will weaken.

Go to a gym, spend less time sitting down, get a standing desk, that kinda thing.