Apparently this one fell completely under the radar! Parishi Pabari, the Global Brand Manager for MLP over on LinkedIn, dropped a post off back 2 months ago celebrating the release of Make Your Mark chapter 6. In it, she included the huge reveal that this is the final chapter.
We still have plenty of Tell Your Tale to go, and it's very possible we will get more G5 horses in other forms on Netflix, but for the series itself, it seems that might have been it! We will keep you posted if anything else appears from this news.