You can get a lot higher population density if you don't assume European climate and agriculture. There's a good reason most people don't live at such high latitudes.
Wet rice farming is almost twice as efficient (in terms of land) as wheat farming and can be used on poorer soil.
It's only slightly more labor efficient, so almost everyone would still need to be a farmer, but maybe they call come into the city for festivals or what-not if that's important. This will require lots of water and either flat or terraced land.
Maize and potatoes are much better than wheat, and grow in similar climates.