The amount you can loan from Duke increases when you take out loans and make your payments on time, it also increases when you sell items back to the stores in the city (not the item market). You will have to sell a lot of things back to the game to get the amount you can loan high.
DPs counted, points and properties didn't. Don't think those mechanics have changed. I believe it's 20%, is the magic number up to a max loan amount of 1b, for just the selling items back to city piece.
example- I sold 5b worth of items to city and can take a max loan of 1b now. Or smaller example, sell 100m in items to city, and you can take a max loan of 20m. And then that max loan can also be increased by making interest payments on time over time.