I am happy to share what i’ve learned about digital painting. Question is, what are you trying to accomplish?
If you want to add texture a textured brush is one way, another is texture layer. If you find a texture you like – you can make it gray scale, adjust levels to limit it to mostly mid-tones and use an overlay blending mode for the layer. Add a opacity layer mask to control where the texture shows through (should be most prominent in the mid-tones). If you are going to use a textured brush – remember to slightly vary the saturation+value or hue+value
If you are looking to make the edges more random (less like cel shading), a texture brush can help – and so can adjusting the opacity & size of your brush followed by multiple passes at the edges (the penumbra and the shadow edge ) – this will make your edges less defined without making them look like cotton. I’d recommend setting the opacity some in the 20% – 50% range and make the brush size pressure dependent. You have to experiment a little to see where the opacity range you are comfortable.
Looking at your image, it looks like you already use hard-edge brushes. I recommend sticking to them. I find soft brushes are good for minor work at the end – like depending shadows, making a “glow” from a highlight or using a blending effect to adjust chroma/saturation (overlay, color dodge, etc.)