Knusperkruste: is there a way to download a specific game version like 2.1 for example? i know you can dowgrade to the former version via the galaxy option but just one version it seems.
No.
You can only download the latest offline installers. Once it has been replaced by the most recent version, the older versions are gone. Even support will not help you out. You should always download & store a copy of an older version & patches if you ever decide to downgrade to an older version. The only exception so far is you can still download version 1.63 since it is a completely different game.
This is what you should do.
Use GoG Galaxy to downgrade your game to 2.1 and confirm it is working & adjust your settings to your liking.
Once you have everything working - make a backup copy of your setting file.
Go to where Cyberpunk 2077 is installed and COPY the whole game folder.
Back up this folder on a different drive or better, 2 copies.
The next time your game bricks, and you want to go back 2.1 and its not available on Galaxy.
Delete or rename Cyberpunk 2077 game folder and
Copy the Cyberpunk 2077 folder of 2.1 into your game folder.
No install required. There is no DRM.
Make sure you disable auto-update in Galaxy. because it will try to update your game.
edit: I think they added a new settings for cpu core usage, change it to windows managed. That might be affecting your fps. a poster stated the new setting is not working.