You can disable SMS on Fastmail after you enable 2FA. I had asked their support, and IIRC the trick is to hold down CTRL when you're on the settings page, which will enable the "Disable SMS" option.
This was years ago, so YMMV.
This was years ago, so YMMV.
It's a real UX issue, the average person just can't back up their TOTP codes at all. Hell, I have a Yubikey as a backup and enroll the TOTP code to both places, and to the Yubikey as U2F.