I could be wildly off base here. Are you UEFI/GPT booting?
GPT: no (MSDOS partition table)
Is the resolution
wrong in both SDDM and in the vttys?
The resolution is wrong in sddm and in plasma launched with startx
from a text terminal (Ctrl+Alt+F2).
I do not know what is vttys but the text shown in the terminal window
(Ctrl+Alt+F2) is fine.
Can you avoid both problems by booting rescue media and choosing
"boot from HD"
or "boot installed system"? If yes, it would seem to me to confirm bootloader
the root of the trouble.
I will create a USB media to test that.