Good thermal paste, lapping heatsinks really does depend on the heatsink/manufacturer some do some don’t.
What might be worth a try is switching off that rear 120 as its competing/diverting the airflow from the heatsink fan and top fan. I found that I was getting a cooler overall setup with just the heatsink fans and rear on mine without using the top ones. Where I did find the top ones to be of use is when I’m hammering the CPU and GPU as a fair bit of air leaks around the GPU instead out of the back, so possibly switch on your rear fan in a similar scenario.
I found out all of this by using a FAN controller in one of the drive bays and trying various combinations and monitoring not only the CPU temp but all the other sensors on the motherboard. Also when I upgraded to a Asus Mobo it’s Fan Expert software came to a similar conclusion.
Your current heatsink fan is not really drawing any air over your RAM so your suggestion of turning it 90 degrees might be worth a try if so then the rear fan will be of more use.
frostytech.com and silentpcreview.com are worth a read.
I’m using a Noctua NH-D14 which works great but its a beast of a heatsink so it needs a deep case.