I use the following options in grub.cfg on installation media to start
autoinstallation,

splash=verbose autoyast=usb:///CTRL/ instmode=http netsetup=dhcp language=en_US
textmode=1 mem=4g

And in the autoyast file, just have,

  <bootloader><global><append>plymouth.enable=0</append>

But, rather unexpectedly found that AutoYaST is also adding "instmode=http
mem=4g" to the installed system grub.cfg.

I would think that the <append/> should override everything from the
installation media grub.cfg. At the very least it should strip 'instmode=http'.