Based on the wording of your post, there’s a few different questions it seems like you could be asking.
How to make this already existing syrup less sweet?
How to make future syrups less sweet?
How to make future syrups with less sugar?
To make the current one less sweet, I think you could add something with bitterness or acidity. They literally make black walnut bitters, and you could look into adding some of that ino the syrup. Depending on how you plan to use it, you could serve it with an acidic component.
To make future syrups less sweet, you could use glucose, which is less sweet than sucrose or fructose. To make them less sweet and have less sugar, you could use many different thickeners depending on how you want the texture. Xanthan gum, gum Arabic, maltodextrin, pectin, etc.