A thicker consistency of the syrup is required when you use it on pancakes or over ice cream thus you can reduce the syrup. If you used honey as a sweetener for your syrup you will lose its medicinal benefits with higher temperature. So I would rather increase the amount of honey to make the syrup thicker than to heat the syrup. Although Rose hips contain some natural pectin, it can happen that the syrup will rather be thicker than loose.