The default font for KDE is Noto Sans 10 pt. 10 pt is really small and the font is fairly wispy at that size. Increasing to 11 pt offers a vast increase in usability and reduction in eyestrain.
To keep the relative size differences, every other font should likewise be increased by 1 pt.
This looks really, really good:
(That Discover's sidebar becomes so huge is a bug that we are working on fixing: https://bugs.kde.org/show_bug.cgi?id=385992)
Compare that to the default settings: