I wonder if you could try a variation that keeps passwords in an existing password manager and just uses this as an alternate UI client -- for example with the 1Password sdk https://developer.1password.com/docs/sdks/desktop-app-integr... or this technique for KeePassXC https://pypi.org/project/keepassxc-proxy-client/ . You could expose existing secrets under an "uncategorized" folder, and add a field like "sklad_folder": "foo/bar" to the secret if the user organizes them.
This way your crypto surface area narrows a lot -- you still need to do the integration securely and be thoughtful about any metadata you cache locally (maybe you don't need any!), but you barely touch actual secrets. And you can freeride on all the edge cases existing password managers handle -- recovery, autolock, sync etc. And you don't need to update passwords in two places. And the trust you're asking from users is less -- if I'm considering using your thing, I don't have to fret about all the little policy things you might have done differently from 1Password, I just have to check if you've made a secure frontend. And I can go partway, open up one vault to the frontend but not others, in a way I clearly understand. I'm paranoid and still wouldn't use a 3rd party client to my password manager, but for people who need this it seems like a much more attractive offer that way.