The biggest fight with this little dude was cooling and noise. When you build small, but powerful, it's loud. I originally had an air cooling system (Arctic Freezer 34 eSports) and I thought it was going to fly out my window even at idle. I took a long shot and compulsively picked up a AIO Corsair H60 from best buy one night. Best, perhaps the only, compulsive buy I've ever purchased. With air cooling I was seeing temps of 45c to 55c at idle and up to 75c under load (65c constant while gaming, BF4/Metro Exodus/Elite Dangerous/Borderlands3). After installing the AIO I've seen 60c only once and that was the original startup before the cooler and thermal paste (included with H60) was fully set in.
Update Note 1/30/2020 I noticed while using Gigabyte OC software that while in AutoTune mode at 3.8 GHz my temps would exceed 90c and were very unstable. Highest recorded was 98c. Once stress test started tuning to 4.0GHz to 4.2 GHz temps dropped dramatically to 54c and became rock solid. I believe while watching stats it was due to voltages being built up and creating excess heat at lower clock speeds. I now have set the software to lock in at 4.2GHz and even at idle my temps remain solid and my CPU cooler never reaches above 68% fan/pump speed under full load. While gaming I observe 46c in BL3 and 44c while playing Arma2 DayZ mod (my personal favorite cpu stressing game). At 7 days of run time before I let it sleep for a little bit and relax I am very impressed and satisfied with the potential of this unit.
As an amateur builder, this was a very fun project for me to build on. I can fit a EVGA RTX 2080 FTW3 Super if I change my power supply to the EVGA 650 SuperNova G3, and plan to do so to see if that changes my temps based on the fact my current 1080FE uses a blower to expel heat out if the case, rather then the 2080 that expels heat into my case. Will update info as I make changes and experiment till I find an optimal set up in this Nano S case.
I've included pictures of a few builds with a 1080 and 2080 just to show different layouts and fitment of parts. The 2080 isn't supposed to fit in this case, but with a "can do" attitude anything it's possible with the right parts. I also found placing a third intake fan (120mm corsair) directing air straight at the 2080 helped temps by about 10c. I believe it also helps move the air up and away from the 2080 to the AIO to get the hot air away and out of the case. I was instructed by Corsair in an online Q/A that the H60 would not have been a capable match for the 3900x, and I do feel it's not enough for full potential with this processor. To keep temps stable a slight bit of tweaking was required and ultimately I set up my gigabyte app center with OC and SmartFan to optimize results.