My severe mouse-clicking induced RSI turned out to be not caused by the clicking itself, but constantly being in a "ready to click" state, i.e: holding your click-fingers(...) slightly tensed on/over the mouse buttons all the time.

Simultaneously this hinders precision movements with the mouse, so my other fingers also tensed up as a result of it.

The solution that cured my RSI for 90% was to route mouse button triggers to keys on an external number-pad, which I operate with my other hand. (I use the "Karabiner" app on macOS to route keys).

In other words: I move the mouse with one hand, and do the clicking with the other hand. This relieves so much tension my RSI was basically gone in a few months.