An update to my problem.
Yesterday I turned the photogrammetry back on, cleared the cache, increased it to 10 GB, switched off Live Traffic, switched off multiplayer (even though I don’t have Xbox Gold or Game Pass) and restarted the whole game.
From that point all of my flying above photogrammetry cities of Paris, New York, Frankfurt, Amsterdam and Vienna was without any major stutters and problems. All the objects were rendered in a pretty high quality and there were no “melting” artifacts.
Now, since I made a multitude of changes to the settings, it’s hard to pin-point which one did the trick, but my guess is on the deletion of the old cache file and restarting the game fresh. I won’t be using Quick Resume any more, because I have a feeling that this also had something to do with the slow performance. 