It occurs to me, on the subject of accents, that it’s really a no-win situation. Germans don’t perceive themselves as having “German” accents, they perceive each other as having Bavarian, Berliner, Plattdeutsch accents and so forth.
Having English-speaking actors with different English accents should only be a distraction to those who think the whole film should have been in German with English subtitles.
There is the argument that the "natural" accent for the portrayal of an educated European of the early 20th century is Received Pronunciation English, as that is what they would be speaking (with a possible hint of their own accent) if they were speaking English. Language teachers teaching Germans to speak like Tom Cruise being scarce in 1920s Germany.