How can I get a syrupy "mouth-feel" in sugar-free syrup?
I don't have a good suggestion for what to use, but at least I can give you some info on xanthan. 

Yes, syrup can take viscosity from an infinitesmal amount of xanthan gum. If you add 0.5% to 1% of the fluid's weight in xanthan, you get a pudding consistency. For a syrup-like viscosity, you need much less. 

But xanthan is not a sugar, and does not make a syrupy consistency. It makes stuff gooey, not sticky. This may be enough for you, if all you need is some thickness, but the texture won't be the same as normal sugar syrup. I haven't tried commercial sugar-free syrup, so I can't make a comparison there. 

Xanthan has also the unfortunate tendency to reduce aroma, although it may not be a problem in the little amounts needed for syrup-like thickness.