A password manager like bitwarden but distributed that would work in p2p to synchronize the encrypted database.
This way we don’t need to install bitwarden on a server or use third party cloud applications (nextcloud) to synchronize the keepass database between our different devices
Why not keepassxc + syncthing?
I like syncthing but if I tell my father or my mother to install it and configure it… he will never succeed ^^ the interest would be to have an application easily usable by everyone without worrying about the technique or having third party applications to install.
I know you said to not use third party applications, but syncthing can do this well. Plus you can sync more than just passwords if it’s useful to you.
Syncthing is capable but not perfect at this. If you edit your password file on device A then edit the file on device B, without syncing in between, Syncthing will have a conflict and you might lose data. Ideally you want the password manager to handle syncing so it can do it at the entry level.