If someone interested - in the experimental build Krita with clipping (test build) - #136 by uvhat with clipping layers support i made an equivalent of blur brush, it is similar to IllustStudio or Сlip Studio blur brush. It is updated to current (5.3.2.1) state.
You can try how it works and think is it feeling close to blur brush from CSP?
This doesn’t touch smudge engine and smudge brushes like in approach current thread, it adds new options for filter blur brush as closest equivalent in Krita.
Example video:
Description of feature: GitHub - uvhat/krita_clipping_layers: Krita mod with clipping layers feature · GitHub
Fast blur brush is a single commit, the code is rather dirty and draft (not ready for merge requests) but demonstrates the main idea. You can build it yourself using 0008-Implementation-of-fast-box-blur-filter.patch from info archive.
The key changes are quite simple - sliding window blur box filter + accumulation of dabs results in the same stroke + link kernel size with brush size to achieve correct pressure control.
I didn’t test it enough, maybe it has some issues. If you find something wrong with it, let me know please. As example, i’m not sure that can works fine with instant preview mode.