Hello Heiner,
There is negligible mainline kernel support for S905X4 hardware at the current time. Amlogic started to upstream the basics but the quality of initial code submissions is poor and developers are slow to grasp the etiquette standards needed so progress is slow. Bring-up is still at the power-domains and pinctrl stages (some UART driver changes were recently posted too, which might help your cause) but there's some way to go before upstream kernels have a usable level of support. We also have to hope the upstreaming effort continues.
Khadas has VIM4 sources here https://github.com/khadas/u-boot/tree/khadas-vim4-r which might help with understanding the changes in the boot process on your board. I didn't spend long poking around, but I can see a lot more use of signed blobs and such. I'm also told OPTEE is now used for firmware loading (Amlogic ships and requires BL32) so there is clear intent to lock things down, and most of the Amlogic driver sources that I can see are subject to an Amlogic license (not a GPL license) requiring permission to disribute the code in source or binary format. That would be a challenge for a distro like LE. In fact, the more I read and poke around into what's needed for S4/SC2/T7 hardware support, the more I think Kodi support will be limited to Android or a few Linux using OEM manufacturers. It looks like Linux support will need NDAs which LE could never sign (as no legal entity) and would never sign (on principle).
Sorry.. not much help 