Will artificial sweeteners work in a coffee syrup recipe?
So, according to a website called A Bar Above, you can make a simple syrup (or "rich simple" apparently) using artificial sweetener, but you won't get the same end result. They tested 3 sweeteners they found, and would recommend sweet'n'low over splenda or equal sweeteners. The taste and aftertaste will be different, and it will depend on the sweetener.

That said, I would NOT expect an artificial sweetener based syrup to thicken like sugar would. You would just need to boil it for a set time and make sure it doesn't boil off or burn. Speaking of burning-- If it's an option, I would recommend getting a jar of instant coffee and mixing that in with the finished syrup instead of making it with actual brewed coffee. Boiling brewed coffee may well give you a bitter, scorched flavor.

My other big concern would just be shelf-life. Sugar actually acts as a preservative in high enough concentrations. (Have you ever accidentally eater years-old hard candy with no ill effect? Thank sugar, lol.) Looking around I don't see any information on how long a simple syrup made with artificial sweetener would last. Although if you use it everyday, you'd go through it pretty quickly...