nodm is an automatic display manager which automatically starts an X session at system boot. It is meant for devices like smartphones, but can be used on a regular computer as well, if the security implications are acceptable.
Install the AUR package.
Now ensure no other display managers get started by disabling their systemd services.
After installing nodm, you should modify the /etc/nodm.conf file.
Now set the NODM_USER variable to the user that should be automatically logged in, and change the NODM_XSESSION variable to point to the script that starts your session. This script must be executable!
Enable the systemd service so it will be started on boot.
sudo systemctl enable nodm