Did you ever consider a perpetual license with yearly upgrades? Essentially like Solidworks model where you buy the software and can use it forever, but the newer stuff you dont get access to if you are out of subscription? The difference I would suggest though is a reasonable up front price and an even more reasonable maintenance plan, like say for Ultra App Engine, $20 all in, first year sub is covered and then following year updates are around $15 a year (or so). Also unlike solidworks if you update 2 years later you get the new stuff, without having to buy in the previous that you missed. That way you are nearly guaranteed someone will update(when you offer them stuff that they want/need for their project). I cant speak for Ultra Engine, as I do not know what all it will offer.
As long as you constantly add in new features and release better stuff consistently around 6 months and 12 months, people would renew. I dont usually buy SDK stuff because of documentation issues but I liked how your software looked and I've been using Unity for last 3 years, but miss C++. I tried a bunch of C++ GUI software and they all sucked, so... glad to find this one.
