This guide is for the deluxe version with 4GB RAM, 32GB ROM. It will also work for other devices, but you will need to find a matching device tree file.
CoreELEC 9.2
First, you need an Amlogic version of UBOOT. You can obtain it in 2 ways:
First, download Amlogic Burn Card Maker and the firmware file. To boot into CoreELEC on SD card you will not need the Android firmware, so it does not matter which firmware version you download, you just need it to run the Burn Card Maker software.
Run Burn Card Maker and burn the firmware to the SD card. When done, the card will contain 3 files, one of which is aml_sdc_burn.UBOOT
Download Ricky Divjakovski's Amlogic Tools 6.0.0. Run CustomizationTool.exe, click Unpack, and select the firmware file you donwloaded in step a. aml_sdc_burn.UBOOT will then be available in folder temp/level 1.
Download CoreELEC, select GenericDevice and your SoC S905X2. Burn it to the SD card with Rufus.
Copy a matching device tree from folder device_trees on the SD card to the root folder of the SD card. Rename it to dtb.img. For the Mecool KM9 Pro g12a_s905x2_4g.dtb works.
Finally, you need to install the UBOOT boot loader. Do not copy the file you obtained in step 1, but modify the boot sectors on the SD card, preferrably on a linux machine:
Mount the SD card and find the device with lsblk. The device found will be sdX.