It’s partially a matter of ratio, you need very little liquid to a fair amount of powdered sugar. Powdered sugar glaze will eventually crystallize like royal icing, so you’re right to think of adding fat or liquid sugar, as those will interfere with crystallization.
American buttercream (butter beaten w/ powdered sugar & flavor plus a little liquid) should form a crust but stay soft. You could add more milk to make it a looser, more glaze-like consistency if you want to slather it over cinnamon rolls (rather than needing it stiff enough to stay on the sides of a cake)
https://www.foodnetwork.com/recipes/food-network-kitchen/vanilla-american-buttercream-3326387
