So, I was wondering if it would be possible to use just the GDM login prompt, but have it feed into KDE desktop and if so what I’d need to tinker with to configure it.
It's definitely possible. As a long time non-KDE/Gnome user I've been using various display managers to log into the window managers of my choice
I think GDM might just have a dropbox somewhere in the corner or the edge of screen where you can choose which WM to spawn after login. So telling it what to spawn should be just a one-time picking
As for the switching from SDDM to GDM, I guess
sudo systemctl enable --now gdm.service -f
sudo systemctl disable sddm.service # might be not needed due to -f