You could either simmer it for longer to further reduce OR thicken it with a small amount of cornstarch slurry. Combine a teaspoon of cornstarch with ½ a tablespoon of water and mix until lump-free, then mix that into the homemade raspberry syrup as it simmers (increase the heat slightly if necessary as cornstarch is heat activated). Stir until it thickens. Add more if necessary.