I'd like to make a cocktail recipe I found which uses a Ginger Cardamom Simple syrup, but I can't take all that sugar. I typically use Torani's Sugar free syrup for drinks (which uses Splenda and some gums for viscosity), and I was wondering if anyone has tried infusing flavors into that syrup. If so did you heat it or just let it sit for X days?
I have never personally tried but my advice would be not to heat it.
In the same way that you wouldn't heat a shop bought sugar syrup - you will boil off the water and be left with a thicker syrup and possibly one which would crystallise.
You can either add the ingredients to your Torani's Syrup and leave for X days to see what happens, or follow a recipe to make your own!
Here is one I found for a sugar-free lavender syrup which you should be able to amend for a Ginger Cardamon version.