Also, as someone with RSI, I find that switching devices help, so use the AMM, one of the more ergonomic Logitechs maybe, a Wacom Pen, the trackpad and so on, switch tools. Also take short breaks, do some physiotherapy and mild regular training to get the muscles to release the tension, if you find the time.
RSI made me a lot less opinionated as to what is a good device design and what's bad as a lot of my favorite devices became painful to use. It sounds like you may have had a similar journey where in your search for comfort, you were motivated to put in the time and effort to learn and practice how to use entirely new form factors or learn to use old devices in different ways.
I'm not sure if I'd have such a wide ranging perspective on input devices if my hands didn't hurt so much.
The Magic Mouse ultimately didn't work out for me because there were too many quirks for me that added up to real issues, but I really liked the idea of having a mouse crossed with a trackpad and would love to see someone else's take on that form factor. Remapping typing motions to my mouse was one of the key strategies that helped me beat RSI.
Having oodles of gestures at my disposal in the Magic Mouse and Magic Trackpad were keys way to do that until I found a 16 button mouse with all the buttons perfectly placed to my liking. It's a Roccat Tyon and I store around 30 different actions, shortcuts, and macros in it to cut down typing motions that are difficult for my hands.