I don’t quite remember how to get the TOTP secret from the Steam app (they could in fact take notes from GOG here), iirc you have to extract it from the Android app via adb;
but once you have it, if this GitHub comment is correct you simply have to set the code size to 5 digits.

If your phone has a rooted Android install, this guide this guide

… I swear when I did it, it wasn’t this hard ._.