Reading along, I thought that adjussting the load might help the accuracy.
However I thought that it may have needed more powder rather than less.
Another thought was that 50 yards is an arbitrary distance and that another distance may produce better results.
Why 50 yards and not 40 yards, or 35 or 45 yards is what I mean.
However the improvements to the accuracy seem to be doing absolutely fine with what you've done.
A different powder or granulation can also help make a difference.
Even if you've tapped out the accuracy, congratulations are still in order.
Keep up the fine work and thanks for the reports.