I won't come at you, for lack of time and expertise. I'm genuinely interested in that approach, but I'm probably too afraid to design an algorithm that would be vulnerable for reasons I couldn't imagine.
For digit-only, I meant platforms that forces you that requirement - my bank does.
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+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.
For digit-only, I meant platforms that forces you that requirement - my bank does.
What about MFA?