Getting the source code is only one piece of the puzzle, before you do that, have you checked if the bootloader is unlockable? Because if you can’t unlock the bootloader then there’s no point trying to get the kernel sources.
You should also bear in mind that if you unlock the bootloader, you’d be degrading Widevine to L3, which means the video quality of Netflix and other streaming services would be downgraded to SD. One of the main reasons why people buy these TV boxes (instead of using a Raspberry Pior something) is to watch streaming services - but unlocking the bootloader breaks the DRM, which would defeat the whole point (IMO) - unless you don’t plan on using those services?