If you can't find prune plums, you can definitely try making this with other kinds of plums (or other stone fruit), adjusting the sweetness as needed.
If you aren't a fan of cardamom, you can leave it out altogether, or experiment with using other flavors in its place. A few I considered were lavender, ginger, or vanilla. For lavender or ginger, I suggest following the recipe as written (using maybe a tsp. of dried lavender or a few thin slices of ginger), but for vanilla I would add a pod (cut and seeds scraped) into the pot at the beginning.
This syrup is great in iced or hot tea. For the latter, I like to add just a tablespoon or two to one mug at a time. The syrup is also great over yogurt or ice cream, or stirred into sparkling water for a homemade soda. Come to think of it, you may want to mix some into iced tea, and save the rest in the fridge for other uses!