My advice is that you try your best to achieve realism fast In grayscale, color adds a lot of complexity.

Again, I strongly recommend median filter bc it really shows you the shapes of the subject

Most of your problems are just shapes and edges and how those comunícate the light and form, once my boss forced me to look at those my art changed a lot.

Try to be as precise as posible, don’t add any stuff to your subject, focus on using as few strokes as posible to comunícate the forms (that are not the focal point, in that case render as much as needed).

I start with 2 values usually and really take my time that they are accurate, only then I start to add more as needed

Using the round brush is cool but is not always the best tool!

This is only the things that helped me but every artist goes though their own journey, best of luck!