It would be very helpful for keto diet stuff too, since breads are basically most of the carbs and calories you eat in a lot of meals
I do worry that much mass not being digested would be trouble for you though.
Depends. If it's sugar alcohols they can be a problem. If it's basically fiber, it depends on your microbiome if you do well on those fibers or not. (Also, some fiber like charcoal can be constipating.)