Chai, for me, is like a kid of espresso- I brew the tea very strong. But unlike coffee, which has it's own bean fats and oils to help the taste and to produce crema, tea is a leaf and doesn't taste as nice when very strong. I suspect also part of the problem was using heavy cream instead of milk, too.
I'll come up with a good formula eventually, but my initial experiment with stevia was less than encouraging. I was curious about how stevia would perform, because five to ten years ago this ingredient was illegal to use or obtain, and we never had the opportunity to research it in the tea lab.