I wonder why to put ingredients into syrup.
When you give a proper dosage to bees, they suck syrup so quickly that no mold can generate. ...and keep feeder clean.
Often guys are mad to avoid chemicals but now it is nice to feed them. When we talk about varroa, give nothing, but normal feeding, give what ever comes into mind!
In our country thymol is popular stuff to prevent syrup fermenting.