How to set a default user on the login screen without automatically logging in?
You can activate the "require password" option in system preferences:



Whenever you are done working you can then just close your MacBook or set it to sleep (instead of logging out or shutting it down, which I assume is what you normally do). If you open it up again after the specified time you are prompted to enter your password, but you are still able to switch user.

Achieving this behaviour after a complete shutdown however is not possible, i believe.

EDIT: Maybe there is a way to achieve this after all, since that is precisely what happens after the installation of a macOS update (that required reboot) is finished. Maybe a specific boot flag that can be set or some config in a plist file...