F3 at boot of the installer only sets 1024x768 for the installation process itself. 800x600 usually means required software either doesn’t exist or hasn’t been installed. Absence of /dev/fb0 would confirm such an absence.
Use the ‘e’ key when the Grub menu appears to remove “splash=silent quiet nomodeset showopts” from the kernel cmdline so that you can see the boot messages and maybe spot an error message. If it doesn’t help, do the same thing, but append “plymouth.enable=0” before proceeding. Any of these temporary changes can be applied to /etc/default/grub to make them default when grub update occurs. You definitely do not want nomodeset unless you are troubleshooting (IOW, temporary) or following instructions for installing a proprietary video driver (commonly NVidia). FOSS drivers for Intel, AMD and NVidia GPUs all require modesetting to be enabled. Do you get output from
ls -l /dev/fb*
If the native mode of your display is 1024x768, it should be picked up automatically by both the kernel and Xorg, but if necessary drivers and/or firmware are missing this function can be blocked. What is the output from the following (install inxi with yast or zypper if necessary):
inxi -G -c0
??? Post here /var/log/Xorg.0.log if it exists, or if it doesn’t, /home/<username>/.local/share/xorg/Xorg.0.log, using CODE tags, or upload it to susepaste.org and provide a link to it here.
While it is common to see on the Internet recommendations and discussion how to specify modelines when EDID fails, it’s not particularly complicated to provide Xorg usable specifications so that it can do its customary automatic modeline calculation and selection without any help from manual (CVT or GTF) mode calculation and specification, e.g. using an /etc/X11/xorg.conf containing the following or similar, depending on the actual display specifications and desired mode: