How do you enable this without first giving google your phone number? My understanding is you have to set up SMS 2 factor auth first before you can change it to TOTP.

On a Workspace account you only need U2F token emulator (https://github.com/danstiner/rust-u2f woks fine) and thenn you can setup u2f first and add normal TOTP in second step. But u2f must stay there. I don't have a personal account to try if it works the same.