Thank you, I try and try and try...
In the meanwhile, is there someone to clarify for me what does that scrambling?
I think is Plymuoth, but I fail to fix it.
On one hand, if I go to the KDE setup for Plymouth, I can't apply any change, it complains about not being able to authenticate to set it up.
This is weird, as I did not enable anything related to it manually,or by mistake as user, the setup formatted itself the partition, so it should be nothing broken from the old config, which, btw, worked.
That lets my setup a bit broken, as grub and any console after logging in is small and ugly.
And I need to press F3 in order to see the messages during boot, in a low resolution screen.
It worked before as full hd (despite the monitor is ultra HD, I did not dare to push it, but worked 2K with nouveau once).