Great to hear that PCIe 4 is not a necessity at this point.

About the PSU, for me the only reason to go with a 1000W PSU is that the fan will remain quiet with a GTX 1080 when idle and gaming as well. At least thats what I hope.

If the same is possible with the 3080 is more questionable, especially if the card draws 320W at all times.

That capability exists even on smaller power supplier, for example the Corsair RM-650 has it; powering the fan is done based on temperature (and that depends a lot on the load), not on the power supply max rating. This means you need to just add your max CPU consumption (not TDP, that is lower), GPU max consumption, the rest of the components and add a 200W reserve, not a 500W reserve.

PSU efficiency is low at the ends of the curve, underutilized (<20%) the efficiency can drop as low as 70%. The top (not peak) efficiency is somewhere in the 30-80% load zone, that is where you want to be.

I was under the impression that it was the power draw alone that decided when the fan kicked in. But it makes sense that the temperature also matters.

Based on that info and that the system will draw about 430W for CPU+GPU, it seems that the fanless Seasonic TX-700 is better suited.

It is quite expensive, but as someone who really hate unnecessary noise, it looks like a good choice.

Changing all the existing cables to a new PSU sucks though :p