But what if you need to use a mouse? Then what I've found best ergonomically is to use a "rollermouse". But I don't feel that works well with a split keyboard, and then I prioritize the mouse. (For me my elbow is the biggest issue)
Personally I think a mouse is not replaceable by anything else that even gives remotely the same productivity.
What can be improved is how often you have to reach for your mouse and how much your hands have to move on the keyboard.
I have personally switched to a tiling window manager that allows me to do much with the keyboard, added the surfingkeys plugin to chrome to allow the same when browsing and switched to using the neo2 keyboard layout (this is for Germans, Dvorak or something similar might be better for you).
The combination of these factors has massively improved how much I need to reach for the mouse throughout the day.
I agree with this. I wrote a script that will focus my most commonly used windows with keyboard shortcuts. Super+F for Firefox, Super+T for terminal, Super+S for Spotify, and so on. Between this, vim, Surfing Keys for Firefox, and just an interest in learning keyboard shortcuts I have mostly eliminated the mouse. I still use it when it's the right tool for the job, but I can get by with far less mousing these days.
I found what caused my RSI was going from keyboard to mouse over and over. Getting a keyboard with a trackpoint so I could keep my hands static fixed it. I would eventually like to get a split keyboard with the thumb trackpoint.
I have tried a bunch of things for this issue. I tried placing a trackpad between the two halves, I tried it on either side as well. It didn't really work for me.
What I landed on is a) mapping the mouse to an extra layer that I use when I only need minimal movement, b) using VIM extensions for as many programs as possible, including my browser and c) just using a normal mouse otherwise.
The Moonlander default layout has a layer that emulates mouse controls. It's not great for long sequences but can help with some small tasks where you prefer not to lift your hands from your keyboard.
For those of you with one at home: hold ';' and then control the mouse with E,D,S,F and left and right arrows left and right click respectively.
I overkill. Mostly I have a trackball in the middle as well as an ergonomic Penguin mouse on the side. I rotate and swap between what is on the left, right and middle. A trackball doesn't need a lot of space.
But what if you need to use a mouse? Then what I've found best ergonomically is to use a "rollermouse". But I don't feel that works well with a split keyboard, and then I prioritize the mouse. (For me my elbow is the biggest issue)