Since there is no mainline linux kernel support for the s905x4 that I am aware of, I suspect that coreelec is using the android based kernel.
Note that currently the boot environments for coreelec and armbian are incompatible. So if you install coreelec, you will need to reflash android in order to use armbian.
Also note that the armbian amlogic environment uses the native firmware (android installed) boot loader.
You have an ambitious task ahead of you, good luck.
Since Armbian is community based, anything want to submit to the project as a result of your efforts would be welcomed. If you need advice on PR submittal, I'd be happy to help.