Long-time Fedora and Ubuntu user trying out Manjaro. First tried the KDE version and found it a bit… much. I think GNOME is more my speed.
Trying to make my login screen not remember usernames, and have found guides to other Linux OS, like Fedora and Ubuntu (sorry, but I cannot post links)
But have not found a solution for Manjaro.
I would not call my self a poweruser, but somewhere in the average range.
Gnome edition uses GDM as the login manager, there is a option to hide the username according to the arch wiki. To me this sounds like your use-case and could fill the role of hiding usernames if you know what users are on the system.