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?
Security is only one part of it. If you host a password manager yourself then things like availability, backups, disaster recovery and monitoring also become your responsibility. I’m hosting my own vaultwarden but there is only a very limited amount of people I would suggest self hosting a password manager to, because I know they have the knowledge to do it and understand the risks.
Since every client of Bitwarden makes a copy of the whole database on the server when it syncs, it’s not like all your credentials are lost when the server gets unavailable. You can make an export of your database on that client and import it on another instance. This said you already have a built-in backup feature.