Changing thick lines to thin

[Ofnuts]

Completely different techique:

• Color-select the black
  
• Select > Shrink by 2px this removes from the selection all the bits that are 2px thick or less. So lines of 2px or less won't be affected.
  
• Select > Grow by 4px: this grows back (and extends a bit) the selection around the thick parts
  
• Filters > Generic > Dilate so shave off one pixel of black
  
• Repeat from 1st step until happy.
  

5 iterations, plus final Gaussian Blur plus Brightness-Contrast: