I've never had 'proper' RSI, but I solved all my wrist strain woes by simply placing the mouse smack bang in front of the space key, at a 270 degree angle, so moving the mouse to my left translates to the pointer moving up.

It gives my right hand a much better position, it's quick to switch between mouse and keyboard, and it gives my elbows room to rest on the table.

The only downside is that it requires a slightly deeper desk than having the mouse next to the keyboard, but you probably want space on the desk for your arms anyway.

I'm using a linear columnar split keyboard now (moonlander from zsa, redox from falbatech before that). That has done much more to reduce strain than anything else I tried.

I would also highly recommend getting a trackball, personally I'm a programmer and have to rely on my hands despite having RSI and CTS.

I was getting RSI constantly no matter what ergonomic keyboard I had (MS sculp or whatsitsname was the only one bearable), eventually I got a programmable split keyboard, ZSA Moonlander - RSI immediately gone. But apparently that was even an overkill, I loved the programmable part so much that I decided to buy Planck for my gaming pc because it's tiny and can fit on my couch-mouse-pad setup... No RSI either! I use it for programming too now. I have a theory that what was hurting me was the need to move fingers to backspace, ctrl, etc. And maybe the staggering as you need to twist your wrists a little all the time. Ortholinear feels just... more sensible, for muscle memory... From now on I buy only ortholinear programmable keyboards, they are fun, make more sense, and don't get me RSI!

A number of years ago I ditched mice for a Logitech Tracman and it made a lot of difference in how my wrists felt each night.

I tried different trackballs but ended up using cheap vertical mice, helped a lot with shoulder pain

The Logitech MX Vertical is the GOAT mouse for these issues, IMO. I picked one up when my Microsoft Sculpt gave up the ghost, and I can't imagine ever going back.

The Scuplt was about halfway to a vertical mouse; the MX Vertical is like, all the way there. It was disorienting at first, but I stuck it out for the first weird day or two and I can't say I've ever been happier with a mouse.

I tried most top rated vertical mice, mx included, and ended up with delux wired one. My advise is to try them all before finding one that fits your hand

This company is dedicated to vertical ergo mouses, they offer some pretty neat features: https://evoluent.com/