Just in case you don’t know how Keepass2Android works here’s how it works. You open the browser navigate to a page with a login-form. You select “share” in the browser and select Keepass2Android as the receiving Application. It will then scan for specific entry. If it’s not found you can manually search for it or create a new one which prefills the URL. Once you select an entry it automatically switches the current keyboard to a special keybaord with more or less just buttons for “Username / Password / Custom Field / Go” and also switches back to previous app (your browser). You the press cursor in username field and press “Username” and same for “Password” or any custom field and once you’re done you press “go”. Then the keyboard is switched back to previously used Keyboard.
It supports AutoExec (so Auto-Open), KDBX4, Quick Unlock (with shorter password once unlocked), Fingerprint Unlock (Fully or just Quick), local files as well as native gdrive & dropbox support with synchronizing or any android file picker. In addition there’s a plugin which allows to fill in username & password in most native apps (kind of auto-fill for apps).
So you see Keepass2Android is very powerful and you will have hard time even coming close to this functionality. But how about providing an android file picker which allows to select kdbx from vault, that should generally work. Other 3rd Party products out there also integreate with Keepass2Android this way. Then all you have to worry about is giving access to database and sync-back any changes as kdbx4 support is available it might be the way to go. Of course you also make the vault database available to any other app that can use a Android File Picker. But as you said even if you would share it with an app by accident which sends it somewhere it should withstand any decryption.
Providiing an Android File Picker for Vault would give users the full power of Keepass2Android and save you a lot of development effort.
I don’t know if something similar exists in iOS because I dropped all my iOS interest about 5-10 years ago when I decided that iOS is just not flexible enough for my taste. And now beeing Android User since Eclair (Android 2.0) nothing will bring me back to iOS ever.