Turn it into a password vault Review
Factory reset and later disconnect your phone from the internet, then setup encryption and use it only to store secure passwords like recovery keys or Bitcoin private keys... It's like a cold storage in a safe, but cheaper, more convenient, and possibly harder to steal.
Ranked in these QuestionsQuestion Ranking
Pro Similar or better security than a cold storage
Similar to some physical password vaults, like Password Vault by Hammacher Schlemmer, it's safer than most cold storages because if someone gets your device it's harder to brute force both encryptions than opening most vaults. Also you likely have time to revoke all existing tokens/passwords before the attacker.
Pro Safe against almost any attack vector
Storing for example OTP back-up codes in a password manager connected to the internet (your home computer, LastPass, 1Password...) is somewhat secure but you always risk that a software exploit may give an attacker access to your accounts/data. An offline storage however may not be compromised as there is no attack surface. You only risk something if someone grabbed your device, but even then you are likely capable of revoking any password/token stored on that device very quickly.
Pro More convenient than a cold storage
Not only it's smaller, but it's easy to edit.
Con Device should be robust
Don't use a phone that is so damaged that it may not turn on. Also keep a list of what you stored on it (in case of theft or loss for example).
Con Only for rarely accessed passwords/data
You cannot copy/paste or fill forms that way (as the device is offline). So it's only not viable to store commonly used passwords. It's only good store rarely used critical data, like: OTP back-up codes, cryptocurrencies recovery keys and such.