SUMMARY
My trackball is a Logitech Trackman Marble. It's a trackball designed for people who have RSI in the index/middle/ring fingers (or people who want to prevent RSI), differently from thumb trackballs designed for people who have RSI in the thumb and the brevis muscles.

Here's the manual showing how it looks like and its 4 buttons: https://www.logitech.com/assets/51557/bossa-trackman-marble.pdf

Since I bought it, I've needed to run qdbus commands to configure it on Wayland, as detailed in https://wiki.archlinux.org/title/Logitech_Marble_Mouse#Plasma_and_Wayland

What I do is:

* enable scrollOnButtonDown
* when I use my right hand, that's all that's needed because scrollButton defaults to 275 (button 3 in the manual)
* when I use my left hand, switch scrollButton to 274 (button 4 in the manual)

Rather than have to write qdbus commands, I'd like a GUI setting for these.

I just found out that scrollOnButtonDown was actually already added with https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/2357, currently showing up as "Hold down middle button and move mouse to scroll", so that's one part of the issue down. But I'm still unable to switch the scrollButton to a button that's more comfortable for left-handed mode.

Implementation wise I can imagine a GUI for this and other input actions like:

"Enter the new combination for Extra Button 1:"
| left | scroll | middle | right |
|            +      Input  ...                |

I'm not sure whether scroll button and middle button are the same thing, if they are, it's simpler:

"Enter the new combination for Extra Button 1:"
| left | middle | right |
|        +   Input  ...         |