Update:
After removing the `quiet` and `splash` boot parameters from the GRUB
configuration, the issue no longer occurs — the graphical login starts
as expected after reboot, without additionally delaying sddm start.
This suggests that the root cause is not purely a systemd race
condition, but also related to how early boot messaging and framebuffer
ownership (e.g. Plymouth or silent boot) interact with SDDM and the
proprietary NVIDIA driver. The problem appears to be timing-sensitive
and possibly framebuffer-related, especially in live-build-installed
systems with newer NVIDIA GPUs.
Best regards!
