I also use a small keyboard (Keychron K3) even on my desktop, but once in the past when I switched to a full-sized keyboard, I intentionally switched to left-hand mousing since the extra distance is only on the right side (with the numpad).

I was also hoping the switch would help balance out my shoulders after years of right-hand mousing. It took a few days to get used to, but now I'm basically mouse/trackpad ambidextrous. I'm not sure if it helped my shoulder; I think maybe it did, but too many confounding factors.

Funny. I feel less strain and pain in neck and shoulders with a split keyboard compared to “normal” due to shoulders being more open.

I guess it just goes to show our bodies are different and there’s no universal ergonomically optimal solution.

My ideal keyboard would have buckling spring switches in the body of a Poker 2 for this exact reason. I can use a separate numpad on the far side of my mouse for extended numeric entry, but the far more frequent use of keeping my left over the keys while my right uses the rodent is made much less RSI-prone.

When you had your shoulder pain, was it primarily at the very base of the shoulder blade? That's where it was for me before I moved to a vertical mouse.

No, it was directly in the shoulder joint, like a rotator cuff injury.