No update yet on the power supply fan - still need to test it and cramp a connector to it.
Going further down this rabbit hole, when I was running the system without a psu fan for a short moment I also found the cpu cooler to be a bit noisy as well. Rattles a bit like the valves of a diesel engine. I took it off and replaced it with something very simple (SilverStone NT09-1700). Well, actually I used the oem aluminum block (as its larger), and screwed the SilverStone fan on top of it.
Pros: when idling the diesel noise is gone Cons: on (synthetic) high loads the fan is noisier than oem, plus as there's no shroud also the system heats up quite a bit. With the oem cpu fan the case keeps cool to the touch and it disperses all the heat to the back of the case via the shroud. When using the SilverStone fan however, the case becomes very warm to the touch, and running benchmarks for 10 minutes or so increases overall CPU and case temps to 10-15 degrees Celcius above stock fan. It also seems a logical result, as the heat from the cpu is being blown everywhere, and the case fan in front is not capable to blow all this heat outside the back of the case. This additional heat inside the case also causes my Sapphire RX 6400 fan to spin to screaming loud revs, whilst with the oem cpu fan it stays off (semi-passive) during most gaming.
So... to sum it all up it feels like I'm stuck in a loop. The airflow design by Dell is quite good by default by taking heat through the case from the front to the back. The oem cpu cooler does its work sufficiently too. Unfortunately on lower speeds the fans are quite audible which would be totally fine in a noisy office environment but not so much on my desk at home. The easiest way to fix it, would be just to move the case right from underneath my monitor to the back (or underneath) the desk. But yeah, me being silly_ed and all, I'm okay with voiding warranties so I'm going to tinker a bit further with this system, and see if I can lower its audible noise while maintaining (or preferably reducing) heat.
1. I have ordered a ID Cooling IS-55 which suppose to offer very good (perhaps the best) cooling capabilities compared to any other low profile fan at the moment. By having this large heatsink, I hope I can keep fan speeds low even under high loads. Oh and yes, the fan is probably a bit too large so this means the internal cd/disk caddy has to be removed. However, that seems to be a part to bring structural integrity to case, and also holds the front plastic panel. So I might need to take out the saw for a good hardware hack 
2. Measure twice cut once. The Foxconn OEM case fan is sized 80×0. By measuring by eye, I expected a 80×80×25 wouldn't fit, however, what I didn't noticed until I took the oem fan out, is that it was mounted in a kind of rubber studs. Meaning, the NoiseBlocker 80×15 fan I ordered is unnecessarily small / under-powered. So I ordered a proper Noctua 80×25 fan to replace that (hopefully it comes with better mounting hardware as well).
3. Also ordered that Noctua 60×25 I was talking about. Not sure yet how to use it, but it could be any of the following three ways. (1) Either I'm going to mount it as a replacement of the psu fan (in case my ebm papst fan is not offering the solution I'm hoping for), (2) otherwise I'm going to mount it on the gpu (using some zip ties), so in case even the inside of the case gets hot because of the IS-55 design, it doesn't get too loud (hopefully). (3) Perhaps I could use it as an additional case fan to make sure cpu heat is getting transferred better towards the back of the case.
Keep you posted!
