Superbacked (secrets management) is now free (opens in new tab)

Free as in beer only:

> Building Superbacked from source code or using official releases is allowed for personal use only.

> Unauthorized distribution or usage of this software (including its source code) is strictly prohibited.

You really should use a law firm to manage anything you want passed on after you die.

Trying to come up with your own solution to this is a very "roll your own auth" approach to such an important thing.