HELP: How do I install SDDM or GDM. Is one better than the other?

OK I 'm ready to throw in the White Flag of SURRENDER. I need HELP!!! I have tried n number of install processes to install KDE Plasma Workspaces on my OLD M5A97 r2.0 machine that has been “upgraded” with an Samsung 980 Pro PCIe 4.0 NVMe M.2 SSD 500 GiB drive via an adapter card. ALL the DE, including Cinnamon, work with the EXCEPTION of KDE, which puts me into an endless login loop.

I am desperate. So… My current DM is LightDM, so I am thinking of CHANGING the DM to either SDDM which I understand to be KDE friendly, or GDM. to see if that solves the problem. I NEED HELP!! Nothing ventured nothing gained. Two brains are better than one.

Q.: Which of the two would you recommend me to try first?

Q2.: Is there some special way I need to install either of them, other than systemctl disable LightDM
systemctl enable SDDM (GDM)
— I THINK if I remember correctly. Or do I first need to install SDDM and / or GDM? If I need to first Download and install SDDM or GDM where do I download them from? ANY HELP you can provide would be greatly appreciated.

D’ Cat