Kmix users are requesting this feature from years.
https://forum.kde.org/viewtopic.php?f=19&t=92925
https://www.reddit.com/r/kde/comments/45orry/how_can_you_change_the_volume_incrementdecrement/d06h78o/?utm_source=share&utm_medium=web2x
It was added inside the text config, but Plasma is a graphical desktop environment, so why not adding it inside gui config also?