This is a good question that most of us run into.
What I did (and this is for working brass only not loading the case) was to put one of my older computers on the bench which I also keep load data. It’s hooked up to an internet movie supplier and watch movies as I load.
Also every half hour or so, get up, stretch and walk around.
What has helped me the most is a good chair. I have a very good leather office chair that I bought used at a store that specializes in office equipment. This allows me to change the height of the seat and other portions of the chair. I paid $130 for it and I have seen the like for over 700$.
I have recently done one process (Size) 500 new cases in one session.
At my age <60 but close, I have been thinking about finding a pneumatic or electric press that takes the fatigue off the shoulder and arm. But don’t know of any.
"The difference between stupidity and genius is that genius has its limits" Albert Einstine