I also think the issue could be the mushy MacBook Pro keys. Not sure how to articulate this, or whether this is a known issue, but these mushy keys over time seem “painful” to use. However my problem with switching to non mushy mech kbs was that I had to always reach for a mouse or trackpad. The convenience (and ergonomics?) of the trackpad under the thumbs is great and normal mech kbs loose that.
So I am now going to try the UHK V2 split keyboard with a trackball module.
For shoulder/rotator cuff related injuries resting your forearm without weight bearing will provide relief, so repeatedly alternating mouse and keyboard will probably cause strain in those tissues.
Unfortunately the term RSI is a huge bag to say some non recovering 'chronic inflammation' which is not particularly helpful in these discussions.
* Get separate mouse and keyboard wrist rests.
* Use more than one type of mousing device and switch up from time to time (e.g. trackball, trackpoint, touchpad, graphic tablet).
* Use a vertical mouse with a handshake-style grip as opposed to standard palm-down mouse, which can put stress on your forearm and elbow.
* Look up RSI forearm stretches and take breaks to stretch your arms and hands for a few moments at least every few hours. I'm also a big fan of gyro ball exercisers.
* An ergonomic chair with movable armrests can keep your arms at around the same height as the desk your working at, which also helps.
* Scroll long pages by clicking the scroll wheel and mousing down rather than using the scroll wheel the traditional way.
Citation needed.
When I had RSI the doctor noted that it was only in my non-mouse left hand. They said the moving of my hand between mouse and keyboard provided a relief for my hand, and recommended I move my non-mouse hand away from the keyboard more often.