//But is it? Do jurors really understand what is asked of them with such a term that is open to wild interpretation?//
It is the job of the advocates to make sure that they do. There is no point in having laws which only lawyers can understand.
The phrase "satisfied so that you are sure" has been used in courts for at least two decades. It is simply an explanation of the level of certainty they must hold. Unfortunately "beyond reasonable doubt" is often interpreted as "beyond any doubt at all" which, of course is totally different. So the explanation "...so that you are sure" can help. But the principle should not change.