I made a syrup from ground black walnut and sunflower seeds, but boy is it overly sweet. I do know I could use some form of flour (qinoa, amaranth, millet, all purpose, corn meal, corn starch, etc), but I’m curious if anyone might know of something I haven’t thought of yet.
Based on the wording of your post, there’s a few different questions it seems like you could be asking.
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.