Glad it's working well for you! The black dust is soot, which means that you are not getting complete combustion, and it seems to be quite a bit for a short time in operation. It might be due to the burning properties of your softwood lumber, can't tell from here. Can you get some seasoned hardwood to burn a few loads? If you clean parts of the interior, burn as much hardwood as you have burned softwood so far, and inspect it again, you will get a good idea of the culprit. Either it will be cleaner, indicating the fuel as the issue, or not, indicating some problem with the build.