greetings
i did similar work for a Zidoo Z9X Pro here (same SoC/brand)
i never did get anything working for coreelec on it but it’s partially due to lack of interest and/or motivation due to no DV FEL on S928X
if there’s anything else i can provide i will but i will tell you if you’re not looking to create your own dtb then a dtb from the device itself will not boot coreelec