Sooo....
I have idea why exactly it works now, but I could play for a few hours now with great performance and no crash. Here is what I did:
1. Deinstalled The Sims 3 Basegame & and deleted all documents
2. Reinstalled, adjusted Game Settings.
3. Installed my Savegame and Nraas Mods
4. Reset the city, total annihilate every homeless Sim and all Sims that have shown up in the Error Report. Reset a few Buildings I was suspicious of and finally moved a lot of gravestones into my building inventory to prevent ghosts from spawning (saw them register in the master controller as Sims?)

Anyway, thank you very much for your patience and help.
It´s fantastic that people like you still care about the game and its users in 2022.
Have a great day!