True, building 4.9 kernel is faster I assume because of using gcc instead of clang with 5.15.
SYSTEM script is used for copying SYSTEM squashfs file where most of the image reside. Like all the binaries, kernel modules, … and kodi binary.
It depends on a work if kernel needs to be updated or system or both.
If you compile only kodi then just binary build.CoreELEC-Amlogic-no.aarch64-22/build/kodi-dd0179fe95cf0acc60430f5dea4a4ffdb28e65a5/.aarch64-libreelec-linux-gnu/kodi.bin needs to be copied to device somewhere on /storage/ and kodi restarted.
To disable kodi from the image to be started automatically then service needs to be masked
systemctl mask kodi
and after reboot kodi will not be started automatically.
After you build kodi you need to copy kodi.bin and start it manually
./kodi.bin -fs &
and then to stop it
killall kodi.bin
Something like that 