Apologies to all for diverting to Inkscape.



Caveat - I am no Inkscape expert, ask on their forum. My advice, just experiment. Is it better than Gimp? All in the eye of the beholder.

ref: https://i.imgur.com/nysC2pM.jpg Inkscape does have a nice calligraphic tool, nice smooth strokes (1) Here is a hint, Inkscape uses Gimp gradients and you can click-drag from a folder into Inkscape. (2)
There are lots of filters including an Oil Painting (3) but you do need to open the Filter Editor and adjust values.
ref: https://i.imgur.com/jIk3D2d.jpg Into Gimp. Always best to import an SVG but some filters such as Oil Painting are not recognised. Export from Inkscape as a png for use in Gimp.

Thanks. Probably I can use the Bristle brush itself. I was too confused to gather it the first time.