These are looking really good to me 
I’m at a similar stage in LoD, I have trouble with the 6b charcoal being over generous in its application even with extremely light pressures which can lead to some horrible marks appearing where they shouldn’t. So to counter that I’ve been using the white as essentially a blender on many occasions to push the darker pigment into higher resolution areas more accurately - particularly on the bumpy side of the paper where careful layering is needed to fill the larger depressions.
There are some great pointers on how best to achieve clean gradations here:- Executing Seamless Gradations in Charcoal/Pastel (e.g., LoD Gradation Blocks)
And don’t be over critical of yourself; your gradations are looking great so far!