The software calculated efficiency for the Braumeister is spot on for me if just using it like set it and forget it, so about 65%. I can up that with stopping the pump during mashing, opening the thing and stirring the mash about once every 30 minutes in my 90 min mash. Also by milling the malt a bit finer. But too fine, and I get a stuck mash. With this, I managed around 75, or how much the software sets as standard for pot and cooler method, so I'm happy with that.
I also start the brew with 23ish liters of strike water, dump 6 kg of malt in there and sparge with 5-6 liters at 80C. Boil 1 hr and end up with 18-19 liters in the fermenter with the rest full of trub in the Braumeister (2-3 liters maybe?) I never measured how much is left and I can sparge with 4 or 6L. I just eyeball it according to how much wort is in when I lift the malt pipe.
In your case, I'd say maybe the mash did not go very well.
I wouldn't be too worried about the body. It could be that your fermentation just stopped a bit higher because all that's left is longer sugars, unfermentable, but good for the body. One way to find out...