Great photos!
If the black finned heatsink can be rotated 90 degrees, that would probably be a big win — the airflow would be directly down the fin channels instead of cross to them. I honestly wonder if this is a manufacturing defect. Could apply better thermal paste at the same time.
Also make sure that two of the fans are blowing in, and the other two are blowing out. This is cooling 101, but I’ve seen worse in real life…
Does it run cooler with the case removed?
It looks like the board is elevated a bit, so the plane of the board is at the midpoint of the fans? Is there anything on the bottom of the board that is taking up space? What’s under there?