Dont use a password manager, they tend to get hacked and leak information.
Dont use a random generator, if you loose it, you will never be able to guess it.
Usually 20+ character is a good and safe lenght, add names or words you could remember, adding numbers or dates.
Having helped hundreds of people with lost passwords, my experience is if you use a too complicated password, your will have a hard time restoring it if you loose the note.
If the passphrase would be just the password to your wallet, then disregard above, but since its used with a 24 word seed then its just an additional layer that does need as much security as a normal password would.
