You know I’ve thought of that too, but if the popular password managers fail, then we have such a big problem on our hands that your personal data will seem trivial in comparison to the larger societal disruption.

You could also use a local password manager that doesn’t upload to the cloud and just syncs with your phone.

Also there are lots of guides on Github on how to set up Mac, PC, Linux, whatever as securely as possible. Good stuff to look up.