I was able to somehow fix this exact problem by doing everything in every forum and by going to the Epic Games Launcher > Library, selecting the three dots on the Unreal Editor for Fortnite, clicking Manage and in the Launch Options write: -d3d11
I had also written that line in my Settings > Fortnite > Additional Command Line Arguments. But, it seems that that doesn’t carry over to UEFN.
I also did all the Tdr in Regedit rigmarole. Maybe that also helped?
I updated everything and disabled all firewalls and anti-viruses, no dice. For me, it wasn’t overclocking or overclocking interfaces, or the latest/older NVIDIA Drivers (Game Ready or Studio), or NVIDIA settings, or a different monitor, or faulty RAM, or compatibility settings. It’s just that I had to wrangle the DirectX11 access for UEFN very specifically. Nope, fiddling with [D3DRHIPreference] in the GameUserSettings.ini file didn’t work either.
Hopefully this will help people in the future.