Several Display Managers support auto login. For example :

• gdm : https://wiki.archlinux.org/title/GDM#Automatic_login
• lightdm : https://wiki.archlinux.org/title/LightDM#Enabling_autologin

There is a way to place the secret file (corresponding to the password) on a dedicated USB stick and have a script attempt to Mount it at boot to unlock the partition. If the USB stick is not found, it will revert to the password prompt. Perhaps this is the best of both?

Make sure not to leave the USB stick plugged in, but rather only take it and and plug it in to boot then safely store it once booted, otherwise you are probably defeating the purpose of having an encrypted partition to begin with.

I'll add a link to read more about it shortly.

Edit: here is one example to set it up (including to auto-decrypt ZFS) https://www.youtube.com/watch?v=7xOLxCwdi-I

I tried to do this a while ago with a GNOME system, setting GDM to automatically log me in, but I ended up always getting prompted for my password from gnome-keyring shortly after logging in which seemed to defeat the point. If you use GNOME, you might want to look at ArchWiki's gnome-keyring page which describes a couple solutions to this problem (under the PAM section) which should be applicable on any systemd distro.

I asked this a while back and was told I am an idiot for asking and it is astonishing I am able to even use a computer, which I clearly am not responsible enough to use.

Just kidding (not really) but consensus was "not very easily". I tried a few methods which were complicated and didn't manage to work. Yes, Windows and MacOS do this seamlessly, but nobody has yet taken the time to add a similar functionality to Linux.

On NixOS I did this:

services.displayManager.autoLogin = {
	enable = true;
	user = "kevincox";
};

# Avoid setting up a keyring every time I do a non-auto login.
# https://gitlab.gnome.org/GNOME/seahorse/-/issues/159
security.pam.services.login.enableGnomeKeyring = lib.mkForce false; 

Another approach entirely is to use pam_mount(8) which can automatically mount a disc on login. I use it to mount /home/$USER (obviously this couldn't be used to mount the root fs !!)