Is there any way to create a new layer which is an image of the flattened visible layers?
Basicly the equivalent of Photoshop's Command+Option+Shift+E shortcut in Sketch.
Sure is! Under the Layer menu is Flatten Selection to Bitmap. I've bound mine to Command+Shift+B via the Keyboard system settings.
There's no shortcut way to do this, you could however use the method mentioned above, just make sure you duplicate the layers before you flatten them to bitmap.
2 shortcuts are still better than a whole set of clicks I guess?