When you evaluate Keepassium could you look into if the application honors not contacting anything on the internet? For me this is a major point of feeling secure when using the application and iOS has no way to block apps from accessing the internet.
You can also directly verify these claims using the App Privacy Report. It's a system feature that shows which domains an app contacted over the last 7 days (and how often). And it works for all apps.
Unfortunately, there is no way to _enforce_ the offline behavior of an iOS app, but being able to monitor it is already something.
Otherwise thank you a lot for checking KeePassXC!