Currently I use custom passwords which I write down in a single file for digit-only passwords. But while I was writing my top comment, an idea about converting

Currently I use custom passwords which I write down in a single file for digit-only passwords. But while I was writing my top comment, an idea about converting letters in app/website name to digits and scrambling them came to me. Take a look for the algorithms: https://www.google.com/search?q=historical+cryptography+techniques https://www.google.com/search?q=historical+cryptography+tech... Choose one which you can run in your head, don't use it as-is, insert random chars at certain places, substitute some letters in the app name for others, make sure it looks like randomly generated, and you will be fine. I still use 2FA with Authenticator (on iPhone), fallback set as SMS. Only thing I need is my government issued ID to get a new SIM card if the current one burns.