This is a German Show line type.
I hate their roach backs. It’s not good for trotting, it’s for galloping - generally only greyhound types have roached backs.
You are right...this dog probably stemmed from German show lines, but some of the Czech working lines are starting to look this way, though not as extreme. There are Shiloh Shepherds and King Shepherds that are mixed with Malamute and other things (and also don’t claim to be GSD because of this, they cannot have AKC papers if they are crossbred), like you mentioned...but you can have breeders that have larger dogs that are completely purebred, that is the GSD part of where those Shilohs and Kings stemmed from. My dogs are purebred German Shepherds but are larger, completely originating from German Shepherds in origin, but are much more moderate in angulation than either the American or German show types.