KOReader can change font weight on Pocketbooks, it works with any font.
You can also achieve something similar for builtin software via custom font names.
For example, I'm using
Literata font, which is freely available. Instead of using normal font, you can take medium or semibold variant and rename its file to "Literata-normal.ttf" (or something like this). The same applies for the corresponding italic font.
Then Pocketbook will use bolder version of the font as the basic one.