It looks like NVIDIA does not handle uninstall properly, it leaves its kernel modules enabled:
# lsmod |grep nvidia
nvidia_uvm 45056 0
nvidia 10584064 1 nvidia_uvm
drm 483328 5 nvidia,nouveau,ttm,drm_kms_helper
i2c_core 94208 6 i2c_algo_bit,nvidia,nouveau,psmouse,drm_kms_helper,drm
Run this to disable them:
rmmod nvidia_uvm
rmmod nvidia
Enable nouveau driver (which is also not done by nvidia drivers uninstaller):
modprobe nouveau
Now can run sddm &
Alternatively, you could have restarted the system, but hey, it's not macos.