I’ve got a BrewZilla Gen 4 35L. I don’t know what you’d consider low efficiency, but the unit’s default profile in Brewfather is ~76%, whereas the software calculated roughly 65% efficiency for the batch in question. I’ve got no idea though how that compares to the Braumeister 20L other than the values in Brewfather are rather similar.
What was a first was crushing the grains myself, but mashing on itself went fine. Looking back, I might have wanted to check for starch with iodine, which I even had available. Might do next time. I also might want to add though that I used 13.5 liters of strike water and did what from my understanding is a batch sparge (raising & draining the mash tun, then adding hot water from a second vessel with a jug) with another 15 liters at 80° C. Not perfect for efficiency, I know, but as described my pre-boil gravity was fine. I must just have been to shy on the heat while boiling.

In the back of my head, I have the number of 10% boiloff being desirable, which would match with your 1 hour boil observation.

My last point is that I’m afraid the beer might be too thin as in too much liquid for too few sugars dissolved in it. I didn’t boil off enough water, so I did not concentrate the wort far enough to reach the desired post boil gravity.