I can't really see the problem. Have you left a load of black spot.
I have hardly ever used chemicals and there has been only a couple of times where I felt like you do - could have done better.
Like your photo it was because there was overhanging trees, possibly an area that held damp, not getting the sun at all.
Sometimes 15LPM is just not enough oomph mate. The lads who use chemical will be able to say better than me.
Looks ok and the main thing is that the customer was happy.
Improving the result - maybe that particular set of circumstances it wasn't able to be improved.
You have done the hypo, you have used the turbo nozzle after trying the FSC, with what you have possibly can't do anymore.
Go over it again, let the hypo dwell more?
Rob