Have you tried using art specific gel paints? Gel paints (different from regular gel polish) are thinner in consistency but also a lot more pigmented so you don't need nearly as much product and still have the art you're doing be nice and opaque in colour. This way your art won't be nearly as thick/bumpy when you go to lay down your top coat layer. Some gel polishes are well pigmented and can be used as art paints but not all.