You can have it prompt you for your user/pass but one of the drawbacks is that you have to use an external keyboard since the login manager doesn’t provide an on-screen keyboard.
If you’re ok with that, then you can look into the SDDM config file in /etc/sddm.conf.d/zz-steamos-autologin.conf
Yes, there are several ways to do this. In increasing order of difficulty they are: