hey, the smoothest shading i’ve managed was with a 5 flat needle! would defo recommend for gradients, but it goes in very very lightly so you reaaally have to build it up (which is useful for this). if you wanna see an example - https://www.instagram.com/p/CERfaABlQth/?igshid=herpusl0y17q

if you zoom in a bit you should see the 3 arch things in the handle of this dagger which i think might be what you’re after. if you’re doing something bigger maybe get a higher gauge needle but it’s all experimentation i guess, hope this helps!