You don’t actually have to explicitly install vivaldi-ffmpeg-codecs as that will be pulled in as a dependency for opera. Also, overriding opera will still install opera for you, it’s just that you’re including the proprietary codecs with it.
In essence, just remove these two lines:
...
- opera
- vivaldi-ffmpeg-codecs
(opera.override { proprietaryCodecs = true; })
flatpak
I haven’t really tried the flatpaked opera, but as far as I know KeepassXC integration with flatpaked browsers isn’t very straightforward: