I think your reasoning for your qscale choice is faulty. Take for example the transition from q=4 to q=5. For a (comparatively) small decrease in file size, you’re sacrificing a (however big) amout of picture quality. Think of it this way: If the orange graph was “near 0% at the transition from q=4 to q=5, what would you pick, the better quality q=4 or the worse one, q=5?
So bascially, what I’m trying to say it that your qscale pick is “one-off”, and this choice is “for the worse”. The correct choice in your case would be to pick 4 or 8 or 10.
If your “change in change-in-filesize” or 2nd-order derivative, whatever one prefers to call it, is approaching 0%, then what you get when transitioning from quality q (better) to (q+1) (worse) will net you near 0% of saved file size.