Alternate between using the left and right Alt keys. The ergonomist's rule of thumb (no pun intended) is to use both halves of the keyboard. So if pressing Alt-x, use the right Alt button, etc.

I had RSI issues early in my career and this advice alone really helped. Never got the Emacs pinky/thumb. I recently switched to a MacOS and that is giving me thumb issues with the overuse of the Meta button. I now consciously have to force myself to use a different finger when pressing Meta.

Always remember: You have five fingers - no need to keep using the same one/two fingers to press Ctrl or Alt. It will take time getting your brain used to using other fingers for this purpose.

Oh, and yes: Definitely got lots of ergonomic pains due to mouse use. In fact, I changed my career from "regular" engineering to SW engineering partially to avoid having to use a mouse (e.g. CAD SW). And every ergonomist you'll meet will tell you "Memorize keyboard shortcuts and avoid the mouse as much as possible."