No need for pcg. The procedural tools existed way before that even in .18 i believe.
They have just never been finished like everything else within the engine, but they do work.
Also 8km^2 is pretty small.
Largest I have is 244km^2 and that’s still small but because it was landscape based it is about as many levels as you can cram in before the drawcalls get to be too much when looking at it all.
Not that it matters, but rather sure you have witcher 3 wrong. They were supposedly using tile sizes of around 1km^2 - you really wouldn’t need to partition things for the size you listed.
Same with many of the other games you listed.
Anyway you want to look at the other ones, and probably the ones built on the engine?
Pugb, fortnite, etc.
Maybe even just the Kite demo.
Re Nanite, don’t know.
Usually you do not gain performance from using it.