I'm in exactly the same situation. I couldn't upload the build to TestFlight because of the error caused by the Kingfisher pod using some private third-party APIs. I also couldn't wait for Apple or Kingfisher to fix it. I didn't make a backup. So I tried to downgrade macOS to Ventura, just like you did, but I encountered the same error: "The volume cannot be downgraded." The best and easiest option I found for myself is to create a new volume on the hard drive and install Ventura there. As a bonus, you will also have access to your files from the main volume without any additional effort.