Fill the layer below the line drawing with monochrome

 

Decrease layer density

Erase the area around the line drawing with the monochrome white bucket tool

1 Make it white and put it on the line drawing layer

 

2 Duplicate the layer and make it a selection

 

add a streamline

line drawing

White

streamline

Arrange the layers in the order of

 

Change streamline to vector layer

Made in Senshu → Correction of line width → Thinning with the default value

 

Trace the part that makes the line sharp