SDDM is supposed to remember your previous login choice. So, even if you didn’t see the default session at the bottom left at your first login for some reason, it should have remembered the choice when you selected it on the second one. You may have something wrong with your login manager, perhaps.
If you don’t want to use auto-login, you might try manually setting it via a config file you create, such as /etc/sddm.conf.d/10-display-server-x11.conf or something similar.
In it, put this:
[General]
DisplayServer=x11
Save, and see if that works.