If DHS closes at 7 and buses run for a full two hours after park close, that only takes you to 9 PM. First, and most obviously, you’ll miss Illuminations, and I can’t understand why you’d leave 30 minutes early at that point. More important is that the boat will take you a good half hour to 40 minutes. So, even if you board the boat at 8:30 on the dot and it departs immediately, you will almost certainly miss the last bus. It also depends on where you are in Epcot. Even from parts of World Showcase, you’re probably faster going to the top of the park and getting a bus than going back to the boat and taking that 30 minute ride only to have to transfer to a bus. Consider, if you leave the park before Illuminations, you will be beating much of the crowd and will actually get home to the AKL sooner than returning by way of DHS’ bus station.
By the way, there is also a bus that runs between Epcot and DHS in addition to the boat.