Since you took out the old valve, do you see any cavitation in it? Cavitation occurs at or near the point where the flow is restricted when partially closing the valve. The water when restricted will spray at a very high pressure and erode the area which is restricting its flow. Reducing the water pressure to the entire house or opening the valve all the way will reduce this problem but that in turn can cause other problems like less water pressure at the shower heads

I can't see much into the old valve, just the ball itself and don't
see cavitation that I'm aware of. Presumably what wore out is the
Spindle O-Rings on the spindle that turns and I can't see inside, and
would have to remove the side plug to get to that (and I supposed it
woudn't be easy to get that removed even with the right size Allen
Wrench).