I heard probiotics is good for constipation. I don't believe constipation alone causes bad breath. On TV there is a show called the doctors. They went over bad breath. They were using a product called breath RX and a tongue cleaner. I use a tongue cleaner myself. Brushing the tongue does not help.
Sinus issues can also be a problem as bacteria can hide out in there. Often I feel my sinus mucus becoming thick. The bacteria can make their way down and back into your mouth. No mouthwash will work for this. I was thinking maybe a Neti pot or other sinus irrigation.
You could have gases escaping up from the stomach. I think if you have H. Pylori or other organism in there they can disrupt digession. The bacteria also produce gasses like hydrogen.
I also noted that when dehydrated BB is noticebly worse than at other times.