ArcoLinux uses Lightdm as displaymanager or loginmanager on all its desktops.
Changing to Sddm on Plasma is super easy starting from ArcoLinux 20.2. It takes only a few seconds since Sddm is already installed just not activated yet.
Type this in your terminal and logout.
sudo systemctl enable sddm -f
If you do not like the default look of sddm, then you navigate to the system settings and download a theme you like.