Altering halvah recipe
This is candy.
It is sugar cooked to a particular concentration with other things mixed in for texture and flavor. Low sugar candy is like low fat ice cream- a contradiction.
If you were to reduce the sugar in half compared to the other ingredients the product would have a harder time holding together. It wouldn't take too far before it was simply nut butter. Still tasty but not halvah.
Try it. You might like the outcome anyway and simply prefer a less sweet and less firm but more nutty spread.
You might have to come up with a new name for it.