You are probably aware that there are many different types of RSI. I have used some of the following remedies for some types of RSI with some success. 1. Ulnar tunnel syndrome (numbness and tingling in the 4th and 5th finger of the mouse-using hand). All sorts of computer mice can apparently cause this due to pressure on the ulnar tunnel. Automating all mouse actions by using Autohotkey mouse macros helped very much in my case. Using a mini pen tablet (temporarily) as mouse replacement may also help to some degree. 2. Tennis elbow (painfulness of the outer part of the elbow) from too much mouse clicking. Temporary relief may be obtained by careful (!) Botox application in very small doses by an experienced (!) neurologist. Do not try this at home. This leads to a therapeutic paresis of the affected muscle and consequently to a temporary immobilization of the affected tendon that lasts several months. Please note that the abovementioned procedure will only cure the tennis elbow if the underlying cause (too many mouse clicks) is completely removed e. g. by using Autohotkey mouse macros. Please note that Botox may lead to serious harm if given by inexperienced and/or incompetent persons. Reference: Lin YC, Wu WT, Hsu YC, Han DS, Chang KV. Comparative effectiveness of botulinum toxin versus non-surgical treatments for treating lateral epicondylitis: a systematic review and meta-analysis. Clin Rehabil. 2018 Feb;32(2):131-145. doi: 10.1177/0269215517702517. Epub 2017 Mar 28. PMID: 28349703. 3. Low back pain/shoulder pain/neck pain. I have found that using a reclining chair with an angle of at least 45 degrees backward combined with a high backrest and headrest may help to some degree.
i switch the hand i use the mouse with...also instead of resting my wrist horizontally i turn it like 60degrees or so and only rest the outer part of it
Definitely continue to seek comfort.
If you're on the right track, and previous injuries are healing, you may find that "comfort" changes over the next few months while that healing is happening. Be patient and don't give up.
Look to the professional pianists' advice about posture and ergonomics. Hire someone to smack you with a ruler and holler "wrists UP!" for a while if you need to.
Personally, I suggest losing mice entirely; go to a trackball. I'm fond of this one specifically [1]; but go with what fits your hand and arm and doesn't require you to exert painful efforts to use.
I became a computer addict as a teenager, and was aware that I'd likely be facing autoimmune arthritis problems later. I paid attention to good keyboard, never used a mouse, and have 35+ years of daily heavy terminal use without RSI type problems. The arthritis has arrived and is biting harder now, but most of that i can deal with by slowing down and paying more attention to accuracy. Back to hunt and peck.
Edit: "stretching" ... kneading dough or big wads of silly putty has helped me sometimes. that "cracking the knuckles" feeling in all the little bones that go through the wrist and palm is hard to achieve otherwise.
[1] https://forum.trackballs.eu/viewtopic.php?f=17&t=58