i use glace and don't shoot when wet and most of my cookies have shine. it's not a super high gloss shine, but it is def. not matte. glace does usually dries shiny, again, not a chrome surface shiny, but still shiny.
i use equal amounts of corn syrup and water (not milk, though you can use milk). mix water and PS first, add corn syrup and then once it is combined, i beat on very slow for 10 minutes.
getting shine with white and black and be challenging (black is next to impossible on most days). avoid the wilton white, it textures the dried icing and destroys the shine.
some colors just don't want to dry shiny...very irritating, lol. i struggle with pink and have no clue why. maybe some colors just don't reflect light as well?
generally better shiny colors from americolor instead of wilton.
oddly enough, i find i get a better shine from a thicker flood than a thinner one. don't know why
also, i think temp in house can affect level of "shiny". warm is better with no drafts.
this is photo is glace and was fully dry for over 24 hours and i think is is about as shiny as you can expect.
hope this is helpful!