You know, it’s funny/not funny, but my first/only pair of fully custom boots was Der Dau. This was way back in the late 80s, and I had an extremely similar experience!
I was measured on site and the boots that I received never fit right (for starters the foot was too small??!) and they never made it right.
Every single pair of semi-custom or non-custom boots I’ve had since then has fit better, including a $250 pair of boots I got a few years ago for schooling.
I have always told people ever since not to get them!