Regarding custom font weight: you can imitate it in the default Pocketbook software, but only if the font has more weights included. For example, there's a good free font
Literata with Medium (500 thickness) and Semibold (600) weights.
Regular weight is 400, so if you want to have a bolder default font, just rename "Medium.ttf" to "Regular.ttf" and "MediumItalic.ttf" to "Italic.ttf", removing the original regular weight files. Just make sure to also have Bold and BoldItalic files, so there will be 4 total. This way you can overcome the software limitation.