Figured out a solution, thanks to Fedora 40 Kde Spin X11 doesn't exist - #15 by random0
I used sudo dnf in sddm-x11 --allowerasing to replace the wayland SDDM with the X11 SDDM. I guess the wayland-based SDDM just can’t deal with launching into X11. I’m not sure how anyone else got this working without following this step.
All well, everything works as expected now.