OK Gotta chime in here. I have been using the Logitech MX-500 mouse for over 12 years now...with no ill effects. It may be discontinued now, but can probably be found on EBay. It is a gaming mouse and very easy on the wrist. Never had a mechanical problem with it either. Secondly, I learned a long time ago after countless hours of gaming, that in order for ANY mouse to be comfortable with little impact to the tendons in the hand, is to ensure that the elbow is lower than the hand. If your chair is too high, so as to get a better look at your monitors, it may be better to lower the chair and tilt those monitors down a bit. I highly recommend this mouse.
20 Hours a day??? Even a butter bath would get uncomfortable. I hope you find relief Mark.