Here is another element on my incremental path to improved accuracy.
A big part of accuracy being consistency, I switched recently from a bare crown knot to a plastic bead that covers the knot and allows for the exact same release friction every time.
This has helped. If before I could play catch at 30 yards and someone could catch the ball 30% of the time without moving their feet, now it is closer to 75% of the time. Maybe half would hit them somewhere if they didn't react.
Realizing I do NOT have any sort of natural talent for slinging I continue to look for ways to make it easier. If it were as close as possible to something I can already do - throw a ball or toss a javelin, maybe it would improve my consistency and then accuracy can be worked toward in a systematic way.
per the pics, I show the bead, then the bead as I normally hold it, lastly the bead as I have been experimenting - holding it like a claw, hand as it would be if I were holding a ball. I use a figure 8 and simply wind up and throw, there is no release of thumb in place of opening hand, just open the hand.
I have tried this with the bead and some of my knots and it seems to have helped accuracy yet again. I was surprised that it didn't feel like a weak hold when slinging rocks compared to tennis balls.
Food for thought.