Finnaly found the issue. I've just tried to open Nvidia X Server Settings and it autoclosed, then I've found inxi -G threw some light: I had no renderer, literally N/A.
So I went to /etc/X11/xorg.conf.d/nvidia-drm-outputclass.conf and found it had been reset to just one nvidia Section. I filled it with the mini-tutorial info, rebooted, and done!
About the ksplash issue, I disabled it with no good results. It stays the same way: Tried 5 reboots, and it took every time around 24~36 seconds between striking the Enter key and the desktop showing itself.
Btw, I've tried to check the SDDM log file, but I didn't find it in /var/log/sddm.log path. Where could I find it?