How do I master the complex systems that still stump me at work?

You handle most of the systems now, which means you have already learned a great deal. What is left is the hard core: the high-voltage work, the complex drivetrains. It makes sense those come last. They are the parts that demand the most, and the ones you were wise not to rush. Do not let the difficulty convince you that you cannot get there. Everything you already understand was once just as confusing as this feels now. The complex system is not a different kind of thing; it is the same kind of thing, with more parts. Break it into pieces small enough to actually study, and learn them one at a time. Pick one system that stumps you and go deep on it alone. Read how it works, trace it slowly, and find someone who knows it to walk you through the parts that do not click. With high-voltage especially, respect the danger and never guess. Patience and safety together are how the hardest systems finally open up. Keep at it, and this core becomes yours too.