I don't want to be a buzzkill but I would have preferred a multi-human campaigns, in part to see how Aurora C# handles the load. But oh well, this Gothic Campaign is not something I'll ignore, thanks Steve!
May I recommend Stormtrooper's massive "History of Mankind after COVID19 pandemic" AAR? Over about two centuries, he built up a human empire, initially 0.5 billion, to a total population of more than 35 billion including conquered aliens. His Aurora C# 1.12 savefile is in the POST SCRIPTUM thread.
Load it up, and you will get a very definite idea of what REAL late-game slow-down, both on the gameplay side and the interface side, feels like!
The short answer to your query, based on this - rather definitive - evidence and my own multi-faction AARs, to the question:
"How [does] Aurora C# handles the load" is:
"Temper expectations and limit scenario size, even on the fastest [2021] CPUs. There's a good reason not to allow more than a few spacefaring factions to exist in any game, regardless of the detection and civilian options you choose, and that reason is ever-increasing and - as Stormtrooper experienced - eventually game-killing lag.
His savefile should be an object lesson for us all."