I know I've now bored you with this, but still, constantly, 29/30 C degrees throughout days of standard operation. Temp graph for the last 3 days is a flat line, regardless what the disks were doing. By zooming in I can see that rarely temp increased to 30/31, but 99% of the time it stays at 29/30 .
I find it quite impressive. Air circulation before the fan must have been really, really bad.

All of the above with the server case open on one side and a temporary fan.

Tonight I am installing a proper fan (expecting a theoretical improvement) and closing the case (which I expect will negatively compensate the better fan and worsen the overall situation a bit). My goal is to stay < 35 C degrees with the case closed without having to dremel a hole in it. I will keep you posted as I know you can't wait to hear more .