How can I universally disable titles on hover in Safari?
You can change the delay for all Safari tooltips by setting NSInitialToolTipDelay. If you set a really long time, it's like they're disabled. For example, 10 seconds:
defaults write com.apple.Safari NSInitialToolTipDelay -int 10000

Delete to use the default value
defaults delete com.apple.Safari NSInitialToolTipDelay


Safari stores preferences in its sandbox container and defaults will only look in the container if you don't have a preferences file from a time when Safari wasn't sandboxed (e.g. if you've upgraded macOS and kept your home directory). Check you don't have a ~/Library/Preferences/com.apple.Safari.plist file for defaults to use the one at ~/Library/Containers/com.apple.Safari/Data/Library/Preferences/com.apple.Safari.plist, or manually provide this domain to defaults:
defaults write ~/Library/Containers/com.apple.Safari/Data/Library/Preferences/com.apple.Safari NSInitialToolTipDelay -int 10000