Right now the grease pencil thickness sculpting brush thickens the stroke for as long you hold the button which makes it really hard to control, especially with a small radius.
What would make more sense to me would be to have a certain set thickness applied instantly, that way you could thicken certain strokes easily without making a mess with variable thickness. You could also have the pressure factored in so you could really sculpt the thickness by pushing down harder or softer.