SUMMARY
It is possible to change the amount that the volume is adjusted when using the keyboard volume controls, yet is not possible to do the same for display brightness and keyboard backlight controls. The display brightness control is locked to intervals of 5%, and the keyboard backlight control is locked to intervals of 20%, even when the hardware supports more granular control. In order to get values in between, you must pull up the Brightess and Color widget and adjust the slider by hand. This is inconsistent behavior and also inconvenient.

STEPS TO REPRODUCE
1. Right click on the Audio Volume widget. Click "Configure Audio Devices...". Click "Configure Volume Controls...". Observe  that there is a "Volume change step" setting.
2. Right click on the Brightness and Color widget. Click "Configure Brightness and Color...". Observe that there is no such setting for display brightness and keyboard backlight.
3. Open System Settings > Power Management. Observe that there is no such setting for display brightness and keyboard backlight. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 40
(available in About System)
KDE Plasma Version: 6.1.2
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION
Another super trivial thing: With keyboard backlights that only have 3 states, such as some newer ThinkPads, it makes more sense to represent the backlight setting as something like off, medium, and high, instead of 0%, 50%, and 100%.