Pure black n white with smooth curves

[ociebieda]

I am already using gimp for years, not professionally, but I know most basics.
I also already used google to find proper solution, but all tutorials are not exactly about what I need, so please do not send me links about simple black n white or greyscale conversions.

What I need is to convert colored image to almost pure black n white effect with smooth curves.
I am not talking about greyscale with "desaturate" or 2 color with "threshold".

Original image in lossless PNG.
It has various colors that I want to remove, not making them greyscale.
https://speedstar.itos.pl/gimp/gimp-original.png


Method 1.
I use "threshold" with value 160, which is close to what I am looking for, but the problem is that it makes image pure 2 color, and all curves are not smooth, making no shades between black and white pixels (use pixel perfect zoom withot blurring filters if you are not sure what I am talking about).
https://speedstar.itos.pl/gimp/gimp-threshold_160.png


Method 2.
I use "desaturate" with "average" setting, make duplicate and set layer mode to "dodge", which keeps smooth curves and nicely removes most filling colors, but there are grey fogs that can be noticed on chest, upper leg, and shoe.
https://speedstar.itos.pl/gimp/gimp-desa..._dodge.png


Method 3.
I use "desaturate" with "average" setting, increase gamma (not brightness, because I do not want to touch black color) with "levels" with value 160, increase "contrast" to value 96 (127 makes image 2 color), but I am still getting grey fogs slightly on chest and noticeably on upper leg, and also some lines become too small.
https://speedstar.itos.pl/gimp/gimp-desa...ast_96.png


Method 4.
I use "color to alpha" on all colors, make duplicate and set layer mode to "dodge", which seems to to have better effect than previous methods, but I am still getting grey fogs that can be noticed on chest, and if I use another duplicate and set layer mode to "dodge", then lines become too small.
https://speedstar.itos.pl/gimp/gimp-all_..._dodge.png


Any ideas?
Thanks for any suggestions, and please, lets discuss it like intelligent adults, no useless comments, no childish behaviors.