first of all, thanks for all your unbelievable efforts. Stack is a great plugin, and I try to convince my colleagues to use it for their higher maths courses. I have once seen a paid maths course from the U.S. called ALEKS and played around with it, instantly liking its potential. With Stack, however, schools or groups of teachers from various schools would be able to build something like it without making their pupils pay for it. Yet for some of us even the step from the exercise book to Moodle is huge, so Stack is way out of reach.
Nevertheless, I do use it and welcome your announcement. So for now it's waiting for Santa Claus
All the best,