Found the short from the waterblock!
As some of your earlier suggested, the waterblock was shorting something, and after successful "handheld" waterblock bootup, and gradual tightening on a few screws and inspecting the block indents and PCB, I found that the waterblock was touching the soldering points of the whole row of chokes, the big blocks with LR22 on them in the VRMs.
After adding a piece of card cut from a name card, the 2080 super can be put together with screws and boot, and the indent shown on the piece of insulating card clearly shows impressions from the soldering points.
Frankly, I am surprised not more users have this problem, as the reference card look to use exactly the same chokes with soldering points sticking out.
Attach photos showing the problem and solution.
Other than this, the waterblock performed better than the stock air cooler after all screws are tightened up, as you would expect.
Hope this helps someone in the future.