From Archwiki > xrandr:
Tip: Both GDM and SDDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/, while for SDDM this is done at /usr/share/sddm/scripts/Xsetup. This method requires root access and mucking around in system configuration files, but will take effect earlier in the startup process than using xprofile.
Pepper itself is overrated. At least the black one.