Thank you for mentioning this, I have managed to get a working implementation of brush stroke previews in the Brush Presets docker! It’s an extra option in my custom build of Krita 5.2.14
Any change to brush settings are reflected in the brush’s stroke preview, just like the brush editor.
I chose to stick an approach similar to Photoshop for simplicity, you can still hover over brushes to see their icon or look in the brush editor.
You can see what was involved in making this feature work in the commit for my 5.2.14 fork here: Stroke preview thumbnail implementation · KDE/krita@26851a8 · GitHub
I hope this rough implementation can serve as a stepping stone to making this feature request realized in a future version of Krita. While it feels great to have this working on my own copy of Krita, I’m sure many others would also appreciate this as an optional feature right out of the box.