I read this - don’t know if it will work or not...use at your own risk.
In order to try and enable Sidecar on your Mac if it isn’t on the list, follow these steps:
Connect your iPad to your Mac with a cable
Open Terminal from your macOS Catalina computer
Type “defaults write com.apple.sidecar.display AllowAllDevices -bool true; defaults write com.apple.sidecar.display hasShownPref -bool true; open /System/Library/PreferencePanes/Sidecar.prefPane” into Terminal (without the quotes)