The next Step would be to add the NFC U2F Authentication Option to the Steam Mobile App. Meanwhile they at least could add the option to add U2F Security Keys and to use them instead Steam Guard TOTP. Of course a such change would require users to get U2F Keys and also preferable such ones that supports NFC which will need some time, but in long term it only hardens the security.

Because that app also has the full login info and even allows shopping which isn't allowed for TOTP apps anyway. Because currently this app is a security risk and doesn't comply to standards.

It also could help to make that Steam App comply with the security standards, by removing the TOTP part in it and require U2F authentication via Mobile NFC instead. Also steam already runs on chromium, so this should be a no-brainer. It's defintely time those ancient and insecure TOPT things gets replaced by the new secure U2F standard. Since i use a Yubikey4Nano for over a year now for various Services (Google, Github, Facebook, Dropbox, Bitbucked and Local Clouds) and now purchased a additional Yubikey4 as backup and a YubikeyNeo that supports U2F for Mobile apps via NFC i definitely support this.

Fido2 u2f security key