the step between 0% and 5% is too big (logarithmic?)
On the contrary, the default brightness control in KDE is linear, while the human eye light perception is approximately logarithmic. That’s why the perceived light change on the lower brightness levels is so big, while at the same time the change on the higher levels is barely noticeable.
What, apparently, have been done by KDE though, is there is now a way to control the brightness in 1% steps which have been added to Plasma 6.0 (more on that here — PowerDevil in Plasma 6.0 and beyond - KDE Blogs).
There is also a workaround solution that translates linear brightness control to set logarithmic brightness levels (https://github.com/Type-Here/kde-logarithmic-brightness)
It works for me on the latest Plasma 5 and, according to the author, should also work on Plasma 6.
As a temporary workaround, you can keep shift pressed when changing the screen brightness with your keyboard keys, this will result in 1% changes. (This also works for audio volume.)
I also hope that the mentionned merge request in the other answer will be accepted, because I think it would fix this issue.