Configure automatic login and encrypt your drive. Shows only "enter passphrase" and is actually secure instead of just a cosmetic password prompt.

how will it know which user to pass over to PAM log in? i think the best way would be to edit the gdm source code, hardcode your username into it, remove the output of names, and compile your own. it will be a lot of work though.

I'm not on Debian, but a quick search led me to this wiki link from Arch. Give it a whirl:

https://wiki.archlinux.org/title/GDM#Hide_user_from_login_list

Edit: context of the Arch wiki link, in case better answers can be gleaned from it, I found it on AskUbuntu: https://askubuntu.com/questions/2471/how-to-hide-users-from-the-gdm-login-screen

I dont know of an easy, clean and elegant way of doing what you ask. I wonder what happens if you edit /etc/passwd with the new name, I wouldn't dare to try. The way I'd proceed though is to start a bare repo with my dotfiles, so if anything happens, I'm back in business with minimal configuration changes.

This is a video about bare git repo: https://www.youtube.com/watch?v=tBoLDpTWVOM&t=0

Stolen from this other video: https://www.youtube.com/watch?v=ApkoY-PN0Nk&t=0