So I switched to Kubuntu with this command:
sudo apt install kubuntu-desktop and everything works fine except one thing: the login screen. The login screen is the default one and I don't know how to change it. I tried a few guides but they don't seem to work.
By default, Ubuntu uses GDM3 (GNOME Display Manager) whereas Kubuntu uses SDDM (Simple Desktop Display Manager).
When you added kubuntu-desktop, you may have been offered the option to select either display manager and you may have chosen GDM3.
Now that you want SDDM, simply run
sudo dpkg-reconfigure sddm