One could experiment with a low friction housing liner slid over the inner cable where it contacts the frame to see if this will reduce the lever pull effort.

Der cables often break near the cable's head. Due to the bending back and forth with the shifting. The cable is wrapped around the spool and is played out or gathered in as the lever/spool gets shifted. Think of what a paper clip does after being bent a bunch, it breaks. With the advent of brake lever mounted shift levers and also the smaller size of the cable it's no surprise that what was an uncommon problem (broken cables) now is quite common and Shimano has published cable replacement time/mileage guidelines. If the cable does break the head can become jammed in the internals of the lever and cause the lever to not work well. I wonder if this is what happened to the first owner. Andy