Unfortunately the builtin fonts have not been updated by Kobo for a long time and we are stuck with subpar fonts on our devices.
You could mitigate the issue to some extent by (either/both) sideloading your own fonts and deleting some of the fonts bundled with the firmware (be aware that some of the default fonts are required for the main user interface). As far as i know, it’s not possible to
replace the builtin fonts because the ttf files shipped with the firmware are encoded in an unknown way.
(see this thread:
https://www.mobileread.com/forums/sh...d.php?t=278392)