As explained in this post Slate UI - How can I disable anti-aliasing on fonts? - #14 by Jamie_Dale you can add these two lines either to each separate project’s editor.ini or to the global Engine\Config\BaseEditor.ini file which will affect all projects (preferable in my case).
Also, fonts not optimized for aliased rendering will look worse, so replacing all the font files with different ones in the Engine\Content\Slate\Fonts directory is a good next step.
In the end, while it’s much better than the default, the editor still tries to render fonts in different places at various multiplications of the base font size, so the jaggy/stray pixels still appear near letters and the text does not look perfect anywhere, even at fractional global application scale values. I wish there was also a way to just set all fonts to be 12px exactly, but that’s a separate issue.