well I see a few things to try next time. First Don't head the instructions for the kit. There is little reason to rack to secondary and certainly not just when the ferm starts to slow down. second with all that dark extract you are going to get a less fermentable wort to start with and the addition of malto dextrin will make that even worse.
third don't head the instructions for the kit.
Generally you want to leave an ale in primary for at least a couple of weeks but really you want to monitor the gravity after the first week or so, when you get two readings a couple of days apart that have not changed then it is done fermenting. But even then the yeast is still working to clean up after itself and it isn't a bad idea to give it a little more time before bottling. This can all be done in primary with no need for a secondary at all.