I have Plasma 5.18 running on Fedora 32 on a 15-inch laptop. Native resolution is 1920x1080. So far I've never been able to strike an optimal balance between font scaling and font sharpness. By default fonts are too small. I am talking about 9-10-11 dots on average, which is perfectly legible out of the blue but over a full day of work causes quite some eye strain for me. So I have to scale up by a 106% factor at least. Which of course loses a *lot* of sharpness.

Result is, I am nowhere near the precision I get from the default settings, let alone the precision I get from Windows 10 -- which by the way achieves a far superior balance by default on my machine.

Any suggestion to help me improve things on Plasma?

Instead of scaling all elements, I found that it's often enough to just force a certain DPI for the fonts.
It's in System Settings -> Fonts, at the bottom