Adobe Illustrator – Apply brush to single stroke in appearance panel

Trying to create a graphic style. Basically, it is multiple strokes applied with different widths, opacity, etc. I wanted to see if it was possible to apply a brush to a single stroke within the style – in other words, one of the stroke items on the appearance panel. There doesn’t seem to be an obvious way to do this. Using CC 2021. Any ideas?

Illustrator – paint/draw inside the drawing created from brush strokes, not shapes

I have been struggling with this for last few days and decided to get expert help here. I have drawing like below (original art is more complex) which was created using brush strokes (no built-in shape objects). I am trying to paint/draw inside using brush strokes just like “draw inside” feature of Illustrator. As it works for closed shapes so I thought making sure all lines are connected will suffice, but it didn’t.