Unless there is a way to make it so you can have UI scaling on one screen, but not another, i don't really see the point of trying to use the SP3 as a 2nd monitor when you're trying to use it as a desktop, i.e. docked.
Better to just buy a 2nd monitor and push the SP3 into the corner. That said, it would be cheaper to buy a mini-dp to hdmi adapter and a usb hub with ethernet. Which would cost about $50 instead of the dock's $200. On the other hand, the dock looks a lot better and is convenient to use. Plus it essentially gives you a 2nd power supply for your Surface, so you can leave the original one in you bad and never forget it.