That is essentially, of course, how password managers work!
There's no reason not to write a password down as long as it's securely stored. You probably need embarrassingly little physical security to store your password more safely than many dodgy website solutions!
I _still_ frequently get my _already set_ password emailed to me in plaintext either on sign-up, or when I click "forgotten". (The site is always treated to an angry email encouraging a thorough redesign - though not of course, a 'reply', which would give them even easier access to my password!)
> "Unless you have concerns about physical security, it is fine to write down your passphrases and keep them in a safe place away from your work desk."
https://www.schneier.com/blog/archives/2005/06/write_down_yo...
Have a (logical) salt for all of the passwords. Don't write down that salt.
So, if you found my piece of paper with passwords, you might see something like this:
Etrade - I1999IbmfsaaymIwIbmoi
Gmail - D9cjeawfocsIdkwhtfts4r
But my actual passwords are something like this:
Etrade - I1999ibmfsaaymIwibmoi804WMainStreet
Gmail - D9cjeawfocsIdkwhtfts4r804WMainStreet
804WMainStreet is tacked on to the end of all of them, but you wouldn't know that from looking at the sheet of paper. Only my spouse knows the salt, and it's easy for us to remember, e.g., maybe 804WMainStreet is the address of the first place we lived together. In theory, this is reducing randomness, which might make it easier to crack one knowing the others, but I'm not super concerned about that.
The two most important elements of security for regular consumers are: 1) Use different passwords for everything. 2) Use multi-factor auth when available.
Whatever you have to do to achieve that is better than not doing it.
*And I actually use initialism for these passwords so I don't have to pull out the piece of paper often, only when I forget. In this example, the Etrade password might be derived from "In 1999 I bought my first stock as a young man. I wish I bought more of it."
Ideally, you'd just set "In 1999 I bought my first stock as a young man. I wish I bought more of it." as your actual password :)
But that's probably less true these days. Since they should be hashing the password anyway, why not allow something huge, say up to 1000 characters.
() Unless you either turn that force into the energy required to brute-force the passphrase/keyfile or use it in a way that invokes xkcd #538.
> it is fine to write down your passphrases and keep them in a safe place