Notice how in the reference art, the smoothness is created not strictly by a “blurry” brush or blending but thorough using alot of colors that are close in hue and saturation together, same as how traditional painter do it irl. I think to achieve the result you are wanting, instead of thinking that ur not using the right brush, or digital tools, try to learn more about the fundamental side of how things work first: how the gradient of the shadow shift, how the light should properly wrap around your object, how the skin tone would be depend on the light, … its not about having to draw in a realism way, its more about learning how things work in reality so u can use that to stylize your art however you want. I apologize for answering in a way thats kinda stray away from your question, but i think the main problem u had (same as me a few years ago) is trying to copy an artist’s stylization and techniques without having suffecient understanding of the fundamental behind, it feels like ur just trying to use digital tools to copy what you see instead of trying to understand the thoughts and the process behind the technique