Sorry to see you leave us. Hope to have you back as a user some day. :yes:
Any files left behind by Opera is not actually ever used by any other application. So you can safely ignore them. If you really want to remove everything you have a few options:
You can drag Opera.app from Applications into App Cleaner’s window (not endorsed by Opera, but I’ve used it personally for years).
Or you can delete the following paths manually:
Or you can delete everything by running this command in the Terminal.app (type very carefully or copy directly):
rm -rfv ~/Library/*/com.operasoftware.*