Now that ChromeOS is no longer of interest, we want to boot openSUSE as soon as possible after power on without any keystrokes.
To do this I needed to alter the boot options (GBB flags) as explained by MrChromebox. When I ran the script under openSUSE, the script couldn't read the settings and instructed me to add the kernel parameter iomem=relaxed, or to use a live USB distro. No doubt one layer of kernel protection needs to be bypassed to use this tool. This was fairly easy to do, I just added this option to the end of the GRUB options to the kernel next boot.
Then when I tried to alter the boot flags, it told me write protection was on. The red [WP] should have alerted me to that. I opened up the box and followed the instructions in the Kodi wiki for disabling the hardware write protect, which is a screw bridge on the motherboard.
By setting a boot option I was able to select 1 second timeout at the Chrome boot script, then going straight to legacy boot without requiring Ctrl-l. It's still possible to type Ctrl-d or Ctrl-l in that 1 second but I don't expect to need it.
I also reinstalled the FW_LEGACY firmware to boot the internal SDD first. This can also be interrupted by typing ESC. I discovered that when I had an external USB SDD plugged in, it tried to boot that first.
So now the machine can boot openSUSE unattended. I'll leave it like this and not install the custom firmware for now as that has a minuscule chance of bricking the box. The other reason not to change to the custom firmware at the moment is that it's a UEFI BIOS which means that the openSUSE will have to be resintalled as UEFI requires a different partition layout.