I dont agree with many things apple does at all, and I also think their password manager has flaws like revealing usernames without authentification.
It is pretty handy though, to have a file where the entries are stored unencrypted, and if the password manager detects an entry it prompts to decrypt exactly that field, maybe with a fingerprint.
KeepassDX needs to run in the background and be completely unlocked to even detect apps or password fields.
Do you know any existing app that can do this?
KeePassDX + Syncthing is the best solution.
Use that but its not about that topic. Its about storing unencrypted metadata (or usinh android Keystore for example) and having autofill work always even if the database is locked, and its quickly unlocked just for that entry