question

The Steam login is specific to the Steam service - if the client device does not have the Steam application running, it's not possible for you to get a ticket from that service, to use for sign-in. So you do have to have a login method linked to the player account that you can use from a client that doesn't have the Steam application, before you'll be able to use something device-specific, like the Device ID, to sign into the account. My question would be: Since you're adding Google auth, why not just use that, rather than add an email or username login? The player should be able to sign into their Google account from any device that can run Steam.